Laser machining system

ABSTRACT

A computer controlled apparatus is disclosed for performing a series of laser machining operations on a work piece in an environment comprised of a non-reactive gas with respect to the material of which the work piece is made. The computer controlled apparatus includes a machining chamber for receiving the work piece and means in the form of a pump for directing the non-reactive gas into the machining chamber at a controlled, selected rate. A laser beam is generated by a laser and is focused into a beam along a path to the work piece. An assembly is provided for selectively moving the machining chamber and its work piece therein through a sequence of steps, whereby the relative position between the work piece and the laser beam is changed such that a series of lasing operations and, in particular, welds may be effected on a sequence of selected sites of the work piece. A computer is programmed to control the rate at which the non-reactive gas is introduced into the machining chamber, as well as to the machining sites of the work piece, whereby the selectively moving assembly may be controlled to dispose the work piece to effect the series of welding operations.

CROSS-REFERENCE TO COPENDING APPLICATIONS

Attention is drawn to the following copending, commonly assignedapplications, all/each filed on even data and incorporated specificallyby reference into the instant specification:

(1) "FUEL GRID WITH SLEEVES WELDED IN NOTCHED GRID STRAPS" by R. Duncan,Ser. No. 414,232;

(2) "PULSED LASER MACHINING APPARATUS" by R. A. Miller and G. D. Bucher,Ser. No. 414,264;

(3) "APPARATUS AND METHOD FOR LASER MACHINING IN NON-REACTIVEENVIRONMENT" by R. A. Miller and G. G. Lessman, Ser. No. 414,242;

(4) "STRAP AND VANE POSITIONING FIXTURE FOR FUEL ROD GRID AND METHOD" byR. F. Antol, R. W. Kalkbrenner and R. M. Kobuck, Ser. No. 414,197;

(5) "GRID ASSEMBLY FIXTURE, RETENTION STRAP AND METHOD" by R. M. Kobuckand R. W. Kalkbrenner, Ser. No. 414,198;

(6) "MOVABLE MACHINING CHAMBER WITH ROTATABLE WORK PIECE FIXTURE", by R.F. Antol, R. Kalkbrenner and D. L. Wolfe, Ser. No. 414,263;

(7) "WORKPIECE GRIPPING AND MANIPULATING APPARATUS FOR LASER WELDINGSYSTEMS AND THE LIKE", by R. Kalkbrenner and R. Kobuck, Ser. No.414,262;

(8) "LASER LENS AND LIGHT ASSEMBLY", by R. Antol, R. Kalkbrenner and R.Kobuck, Ser. No. 414,205;

(9) "WELDING PLATES FOR A FUEL ROD GRID", by R. M. Kobuck, R. Miller, R.W. Kalkbrenner, J. Kerrey and R. Duncan, Ser. No. 414,265;

(10) "PLURAL COMPUTER CONTROL FOR SHARED LASER MACHINING", by J. W.Clements and W. D. Lanyi, Ser. No. 414,204;

(11) "GRID AND SLEEVES WELDING FIXTURE AND METHOD", by J. S. Kerrey andR. Duncan, Ser. No. 414,203;

(12) "CALIBRATION OF AUTOMATED LASER MACHINING APPARATUS" by J. W.Clements and J. R. Faulkner, Ser. No. 414,272; and

(13) "RIGID SUPPORT FOR LASER MACHINING APPARATUS", by D. L. Wolfe, Ser.No. 414,191.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention, in its preferred form, relates to apparatus foreffecting a plurality of precision welds utilizing a laser beam whileeffecting movement of the work piece to be welded through a sequence ofcontrolled movements whereby the work piece is accurately positionedwith respect to the laser beam, and each of the plurality of welds iscarried out with a precisely controlled quantity of energy. Moreparticularly, this invention relates to apparatus for welding theelements, i.e. grid spacers, of a nuclear fuel assembly made of avolatile metallic material such as the zirconium alloy known asZircaloy.

2. Description of the Prior Art

The precision laser welding apparatus of this invention relatesgenerally to the manufacture of nuclear fuel bundle assemblies 10 asshown in FIG. 1 of the drawings. As shown, the nuclear fuel bundleassembly 10 is a self-contained unit comprised of a top nozzle assembly12 and a bottom nozzle assemble 14, between which is disposed a matrixof nuclear fuel rods 18 arrayed in rows and columns and held in suchconfiguration by a plurality of fuel rod grids 16. Though not shown inFIG. 1, control rods are included at selected positions within the arrayof nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids16 provide a skeletal frame to support the fuel rods 18 and the controlrods. The nuclear fuel bundle assemblies 10 are loaded intopredetermined locations within a nuclear reactor and, therefore, theorientation of the fuel rods 18 with respect to each other is rigorouslycontrolled.

The precision laser welding apparatus of this invention is, in oneillustrative embodiment thereof, related to the manufacture of fuel rodgrids 16 as shown in FIGS. 2A to 2E. The fuel rod grid 16 is of anapproximately square configuration, whose periphery is formed by fourouter grid straps 22. Each end of an outer grid strap 22 is welded by acorner seam weld 30 to the end of a perpendicularly disposed outer gridstrap. A plurality of inner grid straps 20 is disposed in rows andcolumns perpendicular to each other, whereby a plurality of cells areformed to receive the control rods and the nuclear fuel rods 18. Theinner grid straps 20 disposed along the rows and columns havecomplementary slots therein at each of the points 24 of intersection forreceiving a perpendicularly disposed inner grid strap 20. An intersectweld 32 is formed at each of the points 24 of intersection, whereby arigid egg crate structure is formed. Further, each of the inner gridsstraps 20 includes at each end a pair of tabs 26 of a size andconfiguration to be tightly received in either a top or bottom row ofslots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slotand tab weld 34 is effected along the top and bottom rows formed by theslots 28 within the outer grid straps 22. Further, a plurality of guidesleeves 36 is disposed on the sleeve side surface of the fuel rod grid16 to receive and guide the control rods disposed therein. A series ofnotch seam welds 40 securely attaches the guide sleeves 36 tocorresponding notches 38 formed within the inner grid straps 20. Theprecision laser welding apparatus of this invention is particularlyadapted to perform a series of controlled welding operations wherebyeach of the welds 30, 32, 34 and 40 is carried out. The precision laserwelding apparatus of this invention not only controls the variousparameters of generating the laser in terms of the pulse width, thepulse height of each laser pulse, and the number of pulses to be appliedto each weld, but also controls the sequential positioning of the fuelrod grids 16 with respect to the laser beam. It is understood that aftereach such weld, the fuel rod grid 16 is repositioned and/or the focalpoint of the laser beam changed to effect the particular type of welddesired.

Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44is disposed longitudinally of the inner grid straps 20 in a parallelrelationship to each other. A pair of spacing fingers 46 is disposed oneither side of a corresponding resilient finger 44 and serves along withthe resilient finger 44 to provide a resilient grip of the nuclear fuelrods 18 that are disposed within the cell formed by the intersectinginner grid straps 20. A resilient finger 44a is disposed to the right asseen in FIG. 2C in an opposing relationship to the spacing finger 46a,whereby a nuclear fuel rod 18 is resiliently held therebetween.

The manner of assembling the inner grid straps 20 to each other as wellas to the outer grip straps 22 is shown in FIG. 2D. Each of the innergrid straps 20 includes a plurality of complementary slots 52. An uppergrid strap 20a has a downwardly projecting slot 52a, whereas a lowergrid strap 20b has a plurality of upwardly oriented slots 52b of aconfiguration and size to be received within a corresponding slot 52a ofthe inner grid strap 20a. At each end of the inner grid strap 20, thereis disposed a pair of the tabs 26 to be disposed within correspondingslots 28 of an outer grid strap 22.

As will be explained in detail later, the inner grid straps 20 arewelded to each other by the intersect welds 32 as formed of projectiontabs 48 and tab portions 50a and 50b. More specifically, a projectiontab 48 is disposed between a corresponding set of tab portions 50a and50b when the inner grid straps 20a and 20b are assembled together. Uponthe application of a laser beam to the tab 48 and tab portions 50a and50b, an intersect weld 32 is formed that is rigidly strong and free ofcontamination in accordance with the teachings of this invention.Further, each end of an outer grid strap 22 has a corner tab 54. Asshown in FIG. 2D, the outer grid straps 22c and 22b have respectivelycorner tabs 54b and 54c that overlap each other and are seam weldedtogether to form the corner seam weld 30.

The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side ofthe fuel rod grid 16 to enhance the turbulence of the water passing overthe nuclear fuel rods 18. Further, as illustrated particularly in FIG.2C, the guide sleeves 36 are aligned with cells formed by the inner gridstraps 20 that are free of either a resilient finger 44 or spacingfinger 46, to thereby permit the free movement of the control rodthrough the cell and through the guide sleeve 36.

U.S. Pat. No. 3,966,550 of Foulds et al., and U.S. Pat. No. 3,791,466 ofPatterson et al., assigned to the assignee of this invention, disclosesimilarly configured fuel rod grids of the prior art. Each of thesepatents discloses a fuel rod grid wherein the inner and outer gridstraps are made of a suitable metallic alloy such as Inconel, and theabove identified interconnections are effected by furnace brazing.However, the zirconium alloy Zircaloy is known to have the desirablecharacteristic of a low neutron absorption cross section which allowsfor more efficient use of the nuclear fuel in the utility operation andtherefore allows for a longer elapsed time between refueling by thereplacement of the nuclear fuel bundle assemblies. In particular, fuelrod grids made of Zircaloy have a lower absorption rate of the neutronsgenerated by the fuel rods than that absorption rate of straps made withInconel. The making of the grid straps of Zircaloy requires at leastseveral changes in the assembly of the fuel rod grids. First, it isnecessary to make the slots, whereby the inner grid straps may intersectwith each other, of looser tolerances in that grid straps made ofZircaloy do not permit a force fitting thereof, i.e. to be hammered intoposition, but rather require controlled fit-up to allow "push-fits" ofthe intersecting grid straps. In addition, Zircaloy grid straps may notbe brazed in that heating Zircaloy to a temperature sufficient to meltthe brazing alloy would anneal the Zircaloy, resulting in a loss ofmechanical strength.

Prior to the selection of a particular method of welding, severaldifferent methods of welding volatile materials such as Zircaloy wereinvestigated including continuous welding with a CO₂ laser, pulsedemission of a Nd:YAG laser, gas tungsten arc welding and electron beamwelding. A pulsed electron beam is capable of power densities of up to10⁹ watts/square centimeter with pulse widths in the micro-second andlow milli-second range. However, welding with an electron beam istypically carried out in a vacuum environment which is relativelyexpensive to build and requires a relatively long time to establish thedesired degree of vacuum therein, thus slowing down the manufacture ofthe fuel rod grids. Further, it is necessary to obtain relative movementof the work piece, e.g. the fuel rod grids, in three dimensions withrespect to the electron beam which would require a very complex gridpositioning system. The use of a continuous electron beam providesrelatively low levels of power (in the order of 200 watts) requiringrelatively long welding times and providing very shallow weldpenetrations. The use of a gas tungsten arc was also considered andproved to be unacceptable for providing a sequence of welds in thatafter a given number, of welds, e.g. 25, the arc electrodes requiresharpening to provide the desired fine arc to produce numerous welldefined welds and to avoid damaging adjacent grid straps or vanes of thefuel rod grids. Two types of lasers are commonly used for weldingapplications: (1) the solid state Nd:YAG laser, which uses a crystal rodof neodynium doped yttrium-aluminum-garnet and (2) the CO₂ laser, whichuses a mixture of CO² --N₂ --He as the lasing medium. An inherentadvantage of the Nd:YAG laser is that its emission is in the order of1.06 micron wave lengths, where glass is transparent to its laseremission. This characteristic permits the use of a coaxial microscopewhich uses the same optic elements for both optical viewing and laserfocusing. Further, a pulsed Nd:YAG laser is capable of 400 watts ofaverage power, of a pulse frequency of up to 200 pulses per second andof a peak power in excess of 8000 watts for up to 7 milli-seconds. Suchhigh peak power permits the Nd:YAG laser to produce welds of relativelydeep penetration, thus insuring the structural security of welded strapsof the nuclear fuel rod grids. Such lasers may be operated from a "coldstart" with its shutter remaining open, whereby the weld time isdetermined by the length of time the power is applied to its flashlamps. Such a method of welding is not particularly applicable to aseries of relatively rapid welds due to the laser rod warm-up time foreach weld in the order of 0.8 seconds. Further, optical path lengthchanges occur until a condition of thermal equilibrium is attainedwithin the laser rod. A second method of operation of the Nd:YAG laserpermits the continuous pulse operation of the laser while using itsshutter to "pick off" a fixed number of pulses, thus eliminating theeffects of laser warm-up and ensuring a uniformity of welds even thougha great number of such welds are being effected.

The machining and, in particular, the laser drilling and welding ofZircaloy is described in articles entitled "Pressurization of NuclearFuel Rods Using Laser Welding", by Peter P. King and "ExternalAttachment of Titanium Sheathed Thermocouples to Zirconium Nuclear FuelRods for the Loss-of-Fluid Test (LOFT) Reactor", both appearing in theproceedings of the Society of Photo-Optical Instrument Engineering,Volume 247, ADVANCES IN LASER ENGINEERING AND APPLICATIONS, (1980). Bothof these articles particularly relate to the manufacture of nuclear fuelrods such as those rods 18 shown in FIG. 1. In the article entitled"Pressurization of Nuclear Fuel Rods Using Laser Welding", by Peter P.King, various possible welding techniques other than laser welding aredescribed. In particular, resistance butt welding was attempted butfound difficult to control and reproduce when welding thin walledcladding. In turn, high pressure gas tungsten arc welding experiencedarc initiation and control difficulties at relatively high pressure. Inparticular, the nuclear fuel rods are described as being loaded withfuel pellets and sealed by gas tungsten arc welding in high purityhelium; thereafter, the fuel rods are introduced into a laserpressurization chamber through a gland seal. The upper end cap of eachfuel rod is drilled by a sharply focused laser beam, while the chamberis pressurized with high purity helium. After the laser drillingoperation, the helium rushes through the drilled opening and into therod; thereafter, the drilled hole is sealed by defocusing the laserbeam. In addition to providing the desired pressurized gas within therods, the use of helium within the welding chamber provides a suitableinert gas that will not rapidly oxidize (burn) or contaminate theZircaloy. Further, a totally automatic fuel rod laser pressurizationsystem is described wherein a tape control system of mini-computer isused to advance the fuel rod into the laser pressurization chamber toits laser welding postion at the focal point of the laser beam, to lockthe gland seal, to control the chamber evacuation and introduction ofthe inert gas helium, and to control the pulse laser operation to effectfirst the desired hole drilling and thereafter, the hole sealing.

A similar teaching of the laser drilling and sealing of a nuclear fuelrod is described in U.S. Pat. No. 3,774,010 of Heer et al. This patentdiscloses that the nuclear fuel rod is brought to a single positionwhere it is first drilled and then the drilled hole is sealed. Thus, itis evident that it is necessary to reposition such a work piece or tocontrol a series of lasing operations as would be required to effect theintersect welds 32, the corner seam welds 30, the slot and tab welds 34,and the seam welds 40 of the fuel rod grid 16 as shown in FIG. 2A.Consideration of the number and type of welds required to manufacturethe fuel rod grids 16 indicates that it is necessary to move the grid 16along X and Y axes in a series of steps to effect the intersect welds,whereas it would be necessary to rotate the work piece in the form ofthe grid 16 from the plane formed by the X and Y axes in order that thenotch seam welds 40 and the slot and tab welds 34 may be carried out.

The prior art has recognized the problem of fretting corrosion, whereinthe surfaces of the fuel rod grids 16 and the fuel rods 18 rub againsteach other increasing the likelihood of weld contamination and eventualmechanical failure of the fuel rod grids 16. Fuel bundle assemblies 10including the fuel rods 18 and grids 16 are designed to be disposedwithin the hostile atmosphere of a boiling water reactor (BWR) orpressurized water reactor (PWR), wherein the coolant, typically in theform of water, is super heated to temperatures in the order of 600° F.,i.e. the boiling point of the water coolant is raised by applyingextremely high pressures thereto. Under such conditions, anycontamination, and in particular, fretting corrosion is enhanced. Apublication entitled "Special Features of External Corrosion of FuelCladding in Boiling Water Reactors", by Liv Lunde, appearing in NUCLEARENGINEERING AND DESIGN, (1975), describes the various mechanismsresponsible for fretting corrosion. First, metallic particles areproduced by grinding or by formation of welds at the points of contactbetween the grid 16 and its fuel rod 18. These metal particlessubsequently oxidizes to form an abrasive powder to increase theabrasive action. Finally, the metal beneath the protective oxide layeroxidizes due to the continuous removal of the metallic oxide by thescraping of the surface over each other. In particular, zirconium alloysare particularly prone to the direct oxidation of the metal by thescraping action.

It is readily contemplated that the continued contamination of thejoints between the inner and outer grid straps 20 and 22 and the guidesleeves 36 of a fuel rod grid 16 will eventually lead to the joint'sfailure. As a result, the fuel rods 18 are subject to intense vibrationsdue to the high flow of the water, leading to the subsequent fuel rodrupture and to the release of the uranium oxide into the coolant water.Most of this uranium is absorbed by the ion exchangers, but smallamounts may also be deposited on core components. The release of theuranium oxide into the water coolant further enhances the corrosion ratenot only of the fuel grid 16 but also of the fuel rods 18. The articleby Lunde particularly notes that the welding of grid and rod materialssuch as zirconium alloys in a contaminated welding atmosphere leads tocontaminated welds and thus the problems enumerated above. Inparticular, there is discussed the problem of tungsten welding ofZircaloy and of the adverse effect of oxygen and water in the weldingatmosphere. High amounts of oxygen will increase the hardness of theweld.

A further article, entitled "External Corrosion of Cladding in PWRs", byStehle et al., and appearing in NUCLEAR ENGINEERING AND DESIGN, (1975),particularly describes the effect of corrosion of Zircaloy noting thatat temperatures in excess of 500° C. that the presence of oxygen reducesthe ductility of this metal. The Stehle et al. article particularlydiscloses that the main problem of tungsten arc welding is thecontamination by impurities in the shielding gas, including fuelparticles or tungsten electrode material. In particular, suchcontamination appears in the form of uranium oxide that appears as aheavy white oxide layer on the fuel rods 18. In particular, the Stehleet al. article suggests that the concentrations of water and oxygen bemaintained at below about 20 and 10 ppm, respectively. Though the Lundeand Stehle et al. articles do not deal with the problems of weldinglarge Zircaloy elements and, in particular, fuel rod grids 16 made ofZircaloy, experience has shown that welds produced in a relativelyimpure atmosphere will produce a weld with an initially low degree ofcontamination that, when subjected to the harsh atmosphere of a nuclearreactor, will be particularly subject to fretting contamination. Thus,it is particularly critical that any welding of Zircaloy and, inparticular, laser welding be conducted in a controlled, pure atmosphereto ensure that weld contamination is minimized and will not deteriorateunder the hostile conditions of a nuclear reactor.

U.S. Pat. No. 3,555,239 of Kerth is an early example of a large body ofprior art disclosing automated laser welding apparatus in which theposition of the work piece, as well as the welding process, iscontrolled by a digital computer. Kerth shows the control of laser beamswhile controlling the work piece as it is moved from side to side alongan X axis, horizontally forward and backward along a Y axis andvertically up and down along a Z-axis. Typically, pulse driven motorsare energized by the digital computer to move the work piecerectilinearly along a selected axis. In addition, the welding is carriedout within a controlled atmosphere and, in particular, the pressure andflow of gas into the welding chamber is controlled by the digitalcomputer. Further, a counter is used to count pulses, whereby the numberof laser pulses applied to the work piece may likewise be controlled.

U.S. Pat. No. 4,088,890 of Waters discloses a programmable controllerfor controlling laser emission and, in particular, the control of a highbeam shutter whereby the desired quantity of laser emission is directedonto the work piece. This patent also discloses the rectilinear movementof a carriage carrying the work piece along a vertical axis, whereby thework piece is successfully brought to a position, where a laser weld ismade. In particular, there is disclosed the effecting of a seam weld,whereby the work piece is rotated while the laser beam is directed at aseam between two pieces to be welded together.

U.S. Pat. No. 3,422,246 of Wetzel discloses a laser cutting machine toolincluding a servo system for controlling servo drive motors to drive awork piece along X and Y drive axes respectively. A transducer isassociated with each of the servo motors to provide feedback signalsindicative of the movement of the work piece along its respective axisto thereby ensure accurate work piece position.

Laser machining systems have been adapted for precision work includingthe cutting of semiconductor assemblies. When lasers are applied to suchhigh precision machining operations, it is important that the relativeposition between the laser and the work piece, and more particularlybetween the laser and the work piece holding assembly, be accurately andprecisely maintained. Unlike a mechanical tool, which is brought intoactual contact with the work piece, a laser is removed from its workpiece by a relatively great distance. In such laser machining systems,the work holding means is disposed at a relatively great distance fromthe laser. In such machines, any relative movement between the workholding means and the laser is amplified by the support structure,causing relatively large movement of the laser beam with respect to thework piece and thus reducing the accuracy with which the laser beam isfocused onto the work piece. Relative vibrations between the laser andthe work holding means is serious in proportion to the desired smallcross section of the laser beam at the work piece. With accuraciesmeasured in thousandths of an inch or less, it is seen that if there isrelative vibration, the focused beam will likewise be vibrated, suchvibration being amplified by the structural support system by which thelaser and the work piece position means are coupled together. For thesereasons, vibration has resulted in problems in obtaining the desiredaccuracy in laser machining, especially where laser beams are focused tovery fine points. Such accurate focusing of the laser beam isadvantageously done over a relatively long focal length but increasesthe difficulty of achieving accurate machining due to vibration andshocks. U.S. Pat. No. 3,803,379 of McKay suggests the use of a rigidconstruction of a laser optical system mounting bed and a work pieceholding frame. In particular, the bed is constructed as a hollow box andinterconnected by locator pins forced fit through holes in one memberand being threaded in the other member. In addition, the laser mountingbed and the work piece holding frame are mounted on vibration isolatingpads to support the entire weight of the bed on a rigid floor.

The noted U.S. Pat. No. 3,803,379 also discusses the problem ofmaintaining the intensity of a laser beam at precise levels. Inparticular, this patent notes that when a work piece is changed, it istypically necessary to shut down the laser while a new work piece isbeing installed and thereafter, to start up the laser bringing it backto a desired level of intensity before resuming machining with its laserbeam. In particular, the change of the laser beam intensity will effectcorresponding changes in the machining effect on the work piece. Toovercome this problem, U.S. Pat. No. 3,803,379 suggests that a divertermechanism be incorporated along the path of the laser beam, whereby thelaser beam may be diverted into a heat sink. Thus, while the work pieceis being replaced, the diverter mechanism diverts the laser beam intothe heat sink, thus allowing the laser to keep firing at a uniform ratewithout being shut down so that its temperature, once established underequilibrium conditions, will not be altered between machiningoperations. Further, experience has shown that with heavy laser usage,the intensity of the laser beam will attenuate with time due to aging ofthe laser itself as well as of the excitation lamps associatedtherewith. In addition, the laser beam upon striking a work piecetypically throws off gaseous material and debris that may coat the workpiece or the laser focusing lens, whereby the machining efficiency isattenuated. Thus it is necessary to periodically calibrate the lasersystem, whereby the energy level of the laser beam as imparted to thework piece may be accurately controlled.

In the initial development of laser machining systems, lasers wereemployed for individual, low production machining operations. With thedevelopment of the art, laser systems were increasingly employed forhigh production work processing operations as would be controlledautomatically by computers. As described above, such high productionsystems operate efficiently to reposition the work piece, whereby asequence of welds or other machining operations may be rapidlyperformed. Under such demands of continuing excitation, laser lifebecomes a factor in terms of efficient operation and of cost ofproduction. It is contemplated that under high usage where repeatedwelds are required, as for the production of the above described fuelrod grids, that laser life would be a significant factor to consider.Under heavy usage, the life expectancy of the lamps exciting the pulsedlaser would be in the order of several days, and after this life hadbeen expended, it would be necessary to replace at least the lamps, aswell as to calibrate the new laser system.

In order to improve laser efficiency and life, the prior art asillustrated by U.S. Pat. Nos. 4,223,201 and 4,223,202 of Peters et al.and U.S. Pat. No. 4,083,629 of Kocher et al., discloses the time sharingof a laser beam emitted from a single laser and alternatively directedalong first and second optical paths onto a single work piece. U.S. Pat.No. 4,083,629 describes problems with automated welding systems whereinthe work piece requires a plurality of welds to be made; in particular,the work piece may be brought to a first station, whereat a first welderis operated, and then transferred to a second station, whereat a secondwelder effects a welding operation. Alternatively, two welders could beused at a single station to effect the plurality of welds, thusminimizing the need to transport the work piece from the first to thesecond stations. However, these methods require that either the workpiece be transported or reoriented, thus decreasing the production rate,or that two welders be used thus substantially increasing the capitalinvestment of such apparatus. As an attempt to overcome these problems,U.S. Pat. No. 4,083,629 suggests the use of a bimodal switching means,whereby the laser welder sequentially welds at two distinct weld sites.In particular, there is suggested a motor for rotating a reflectedmirror, whereby the beam is alternately directed along a first and thena second focal path to the work piece to effect first and second weldson a single work piece such as an electrical component. It is apparentthat there is an automated control of the laser welder to synchronizethe firing of the laser with the switching of the laser beams, the wirecutting, and other handling operations. U.S. Pat. No. 4,223,201describes a somewhat similar laser system adapted for larger work piecessuch as would be encountered in ship construction. In particular, U.S.Pat. No. 4,223,201 suggests the use of a rotating mirror to sequentiallydirect the laser beam along first and second paths, whereby a singlelaser beam may be time shared. In addition, a suitable automaticcontroller is employed to control corresponding first and second weldingheads that are moved in time relationship with the beam sharing, so asto effect sequentially a series of welds at two different locations on asingle work piece. U.S. Pat. No. 4,223,202 suggests the seam welding oftwo pieces together with the welding taking place on opposite sides ofthe work pieces at substantially the same point to effect a two sidedlaser seam weld, while the automated controller effects movement of thewelding heads with respect to the work piece.

U.S. Pat. No. 4,078,167 of Banas et al. recognizes the problem ofatmospheric contamination of the weld site during laser welding. Laserwelding in a vacuum has been attempted, but this patent notes thatvacuum welding limits the size and shape of the work piece that can beaccommodated as well as increases welding time required to create thevacuum condition. Alternatively, the work piece may be totally immersedin an inert gas, or a trailer shield may provide a flow of known inertgas such as argon over the area of the work piece to be welded. Inparticular, U.S. Pat. No. 4,078,167 discloses a shield housing forestablishing an inert atmosphere about the weld location of the workpiece as the work piece is transported beneath the shield housing. Aninert gas, typically argon, is passed through a gas passing means havinga plurality of openings therethrough for providing a uniform blanket ofinert gas which flows over the work piece and through a passage betweenthe shield housing and the work piece into the atmosphere. The flow ofinert gas prevents to a degree atmospheric gases including oxygen andwater from flowing into the welding zone. It is stated that the flowrate of an inert gas is controlled to shield the weld from reactivegases, but causes turbulence of the melted material which would produceporous and uneven welds.

U.S. Pat. No. 4,078,167 does not mention the particular metal to bewelded and does not contemplate the laser welding of Zircaloy as for theabove-described fuel rod grid. Zircaloy is known to be highly reactiveto oxygen, nitrogen, and water as found in the atmosphere, and weldingtests leading to this invention have demonstrated conclusively thatinert gas flow around the immediate weld area does not provide adequateshielding for the laser welding of Zircaloy. In accordance with theteachings of this invention, an atmosphere of an inert gas such as argonhas been established with a purity in the order of 10 PPM, which degreeof purity is not contemplated by U.S. Pat. No. 4,078,167.

The above discussion of the prior art illustrates the significantproblems in achieving automated laser welding of a highly reactivematerial such as Zircaloy, wherein the work piece is sequentially movedunder an automated controller to effect a number of precision welds. Asenumerated above, it is necessary to move the work piece, e.g., thelaser weld grid 16 as described above, along each of its X, Y, and Zaxes with respect to the focused laser beam while maintaining anexceptionally high degree of purity of the surrounding atmosphere toavoid contamination of the welded material. In addition, it is desiredto achieve a high degree of laser efficiency, even while the work pieceis being moved through a sequence of positions in three dimensions withrespect to the laser beam. In addition, there are problems of effectingprecise welds in parts of small dimensions and, in particular, ofmaintaining the power level of the impinging laser beam at preciselevels for different types of welds, noting the attenuation of laseroutput as a laser system including the laser rod and excitation lamps isused at high work duty ratios over an extended period of time and theeffect of laser welding debris.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a new andimproved programmable control of laser machining operations of a workpiece.

It is a more particular object of this invention to provide a new andimproved computer apparatus for controlling the laser machining and, inparticular, the laser welding of plural welding sites of a work piece,wherein a sequence of relative movements between a laser beam and thework piece is effected under computer control.

It is a more particular object of this invention to provide a new andimproved laser machining apparatus wherein the work piece to be lasermachined is maintained in an atmosphere of a gas non-reactive to thematerial of which the work piece is made.

It is a still further object of this invention to provide a new andimproved laser machining apparatus, wherein parameters may be selectedso as to control the movement of the work piece to be machined, as wellas to control the rate at which a non-reactive gas is introduced aboutthe work piece.

It is a still further object of this invention to provide a new andimproved programmable, computer controlled apparatus for controlling theenergy level of the laser beam as directed onto the work piece.

It is a more particular object of this invention to provide a new andimproved programmable computer controlled apparatus for controlling theenergy level and particularly the number of laser pulses directed ontothe work piece.

It is a still further object of this invention to provide a new andimproved computer controlled apparatus for laser machining, wherein thelaser beam may be measured to calibrate the power level of the laserbeam incident upon the work piece to a level corresponding to theprogrammed parameter.

In accordance with these and other objects of this invention, there isprovided a computer controlled apparatus for performing a series oflaser machining operations on a work piece in an environment comprisedof a non-reactive gas with respect to the material of which the workpiece is made. The computer controlled apparatus includes a machiningchamber for receiving the work piece and means in the form of a pump fordirecting the non-reactive gas into the machining chamber at acontrolled, selected rate. A laser beam is generated by a laser and isfocused into a beam along a path to the work piece. An assembly isprovided for selectively moving the machining chamber and its work piecetherein through a sequence of steps, whereby the relative positionbetween the work piece and the laser beam is changed such that a seriesof lasing operations and, in particular, welds may be effected on asequence of selected sites of the work piece. A computer is programmedto control the rate at which the non-reactive gas is introduced into themachining chamber, as well as to the machining sites of the work piece,whereby the selectively moving assembly may be controlled to dispose thework piece to effect the series of welding operations.

In a still further aspect of this invention, the computer is programmedto control a beam directing or splitting assembly to first direct thelaser beam along a first path to a first work piece and then along asecond path to a second piece. It is contemplated that two computers areconfigured to effect the laser machining of each of its work piece andare interconnected to coordinate the control of the beam directing meanssuch that only one of the two computers has control of the laser beam atany one instant of time.

In a still further aspect of this invention, the machining chamber ismounted upon a rigid bed that provides a first substantially planarreference surface and there is provided a housing about the chamber forproviding a second substantially planar sealing surface and an assemblyfor adjusting the position of the machining chamber such that asubstantially uniform gap is provided between an upper peripheralsurface thereof and the sealing chamber to permit the movement of themachining chamber without restraint, while maintaining the integrity ofthe machining environment within the machining chamber.

In a still further feature of this invention, the machining chamber ismounted upon a slide table to permit the movement of the machiningchamber and its work piece from a first position, wherein the laser beamis directed onto the work piece, to a second position remote of thelaser beam wherein the work piece may be readily inserted into andremoved from the machining chamber. The slide table may be moved to athird position, wherein the measuring device mounted thereon is disposedto receive the laser beam to provide a signal indicative of its powerlevel, whereby the degree of excitation of the laser may be adjusted orcalibrated in accordance with a parameter programmed into the computer.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of a preferred embodiment of this invention ishereinafter made with specific reference being made to the drawings inwhich:

FIG. 1 is a perspective view of a nuclear fuel bundle assemblyincorporating a plurality of grids made in accordance with the teachingsof this invention;

FIGS. 2A-2E are respectively a perspective view, a plan view, asectioned side view, a perspective exploded view, and a plan view of afuel rod grid made in accordance with the teachings of this inventionand incorporated into the assembly of FIG. 1;

FIGS. 3A-3L show in a series of perspective views the sequence of stepsfor welding the nuclear rod grid as shown in FIG. 2, and FIG. 3M is agraph showing a laser beam profile;

FIG. 4 is a perspective view of the precision laser welding apparatus inaccordance with the teachings of this invention;

FIG. 5 is an exploded perspective view of the structural support systemfor the laser welding apparatus as shown in FIG. 4 and including a mainframe and a kinematic support for rigidly supporting a laser system withrespect to a pair of work pieces, e.g. nuclear fuel rod grids, aspositioned by left and right positioning modules;

FIG. 6 is a perspective, schematic representation of the laser system asincorporated into the precision laser welding apparatus as shown inFIGS. 4 and 5 for directing on a time shared basis a laser beam emittedfrom a single laser source to each of two work pieces, e.g. nuclear fuelrod grids;

FIG. 7 is a side elevational view of the laser welding system as shownin FIG. 4;

FIG. 8 is a partial front elevational view of the laser welding systemas shown in FIG. 4;

FIG. 9 is a plan view of the laser welding system taken along lineIX--IX of FIG. 8;

FIG. 10 is a side sectioned view taken along line X--X of FIG. 8;

FIG. 11 is a side sectioned view taken along line XI--XI of FIG. 8;

FIG. 12 is a partial front elevational view taken from the perspectiveof line XII--XII of FIG. 11;

FIG. 13 is a partial sectioned view of the mechanism for permitting themovement of the slide table as taken along line XIII--XIII of FIG. 11;

FIG. 14 is a perspective exploded view of a welding chamber as shown inFIG. 4 and its mechanism for selectively positioning its rotatablefixture;

FIG. 15 is a front sectioned view taken along lines XV--XV of FIG. 9,particularly illustrating a welding chamber, its mechanism for rotatingselectively its rotatable fixture, and a B-axis rotation drive coupledwith the aforementioned mechanism;

FIG. 16 is a side partially sectioned view taken along line XVI--XVI ofFIG. 15, particularly illustrating the positioning mechanism of therotatable fixture and the relationship of the laser focusing lensassembly to the welding chamber;

FIG. 17 is a sectioned view taken along lines XVII--XVII of FIG. 7,particularly illustrating the laser focusing lens assembly;

FIG. 18 is a plan partially broken away view of the rotatable fixture asdisposed within the welding chamber of FIG. 14;

FIG. 19 is a sectioned view of the rotatable fixture as taken along lineXIX--XIX of FIG. 18;

FIG. 20 is a side, sectioned view of the rotatable fixture as takenalong line XX--XX of FIG. 19;

FIG. 21 is a schematic diagram of an argon supply system, whereby asuitable inert gas, e.g. argon, is supplied from a tank thereof to eachof the welding chambers and laser focusing lens assemblies;

FIGS. 22A and 22B form a schematic diagram of the computer implementedcontrol system for the laser welding system showing the relationship ofthe interface circuits with respect to the central processor unit (CPU)and memory and to each of the chamber positioning mechanisms, a secondlike computer control system, the laser system, the argon supply system,the vacuum exhaust system, the B axis rotation drive, the oxygenanalyzer, the moisture analyzer, and the thermopile;

FIGS. 23A and 23B are respectively front views of the laser welderdisplay panel and the machine function panel, respectively associatedwith the laser power supply as shown in FIG. 4 and the central processorunit as shown in FIGS. 22A and 22B;

FIGS. 24A and 24B are a high level flow diagram of the part programillustrating the steps of the control process whereby the laser weldingsystem is controlled to effect a series of welds of the nuclear rod gridin a precise fashion; and

FIGS. 25A-25R are application subroutines that are bid by M, S, and Tcodes set in part by the part program as illustrated in FIGS. 24A and22B, and FIG. 25S is a curve defining the characterizing parameters ofthe laser pulse forming network.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The fuel rod grids 16 are comprised as described above of the inner andouter grid straps 20 and 22 that are assembled and welded together asshown in FIGS. 2A to 2E. Each of the grid straps 20 and 22 is punchedfrom a continuous roll of slit material and accumulates some surface oilin the punching operation. The oil film is cleaned and thereafter, thestrap is annealed and then assembled into a work fixture as described incopending application entitled "GRID ASSEMBLY FIXTURE, RETENTION STRAPAND METHOD" (Ser. No. 414,198). Thereafter, the grid 16 and fixture arewelded by the laser welding system 100 of this invention which carriesout each of the intersect welds 32, the corner seam welds 30, the slotand tab welds 34, and the notch seam welds 40 in a pure atmosphere of aninert gas. Referring now to FIGS. 3A to 3L, the sequence of the weldingsteps in the inert gas is described in accordance with the teachings ofthis invention. The laser welding system 100 will be described in detaillater; it is believed that an understanding of the manner in which thework piece, i.e. the fuel rod grid 16, is manipulated in each of threedimensions will facilitate an understanding of the operation of thelaser welding system 100. As is apparent from these drawings, the fuelrod grid 16 is incrementally moved along its X and Y axes within a planeand is selectively rotated about its Y axis. Significantly, theaforementioned motion is carried out within a chamber wherein theatmosphere of the inert gas is maintained to a high degree of purity.The first step is illustrated in FIG. 3A, wherein the fuel rod grid 16is disposed within the controlled atmosphere as formed by the weldingchamber with its vanes 42 extending upwardly. A welding fixture isdescribed in the copending application entitled "WELDING PLATES FOR AFUEL ROD GRID" (Ser. No. 414,265) whereby the inner and outer gridstraps 20 and 22 are fixedly disposed with respect to each other duringthe welding operations. A vane suppressor fixture is a tool that is usedfor deflecting the vanes 42, whereby the vanes are fitted within thewelding fixture; the vane suppressor fixture is described in thecopending application entitled "STRAP AND VANE POSITIONING FIXTURE FORFUEL ROD GRID AND METHOD" (Ser. No. 414,197). The atmosphere is purifiedby directing argon gas into the welding chamber until the desired degreeof purity is reached, i.e. 10 ppm of water and 7 ppm oxygen. When thepure atmosphere has been established, the fuel rod grid 16 is moved in aseries of incremental movements along the X and Y axes, whereby each ofthe points 24 of intersection between inner grid straps 20 is alignedwith a laser beam 178 and thereafter, a controlled amount of energy isimparted thereto to effect the intersect weld 32. As will be explainedin detail later, the laser beam 178 is provided by a pulsed Nd:YAG laserthat is excited by pulsed excitation lamps energized by a calibratedreservoir voltage to deliver a specified level of energy to the grid 16.In particular, the number of pulses directed onto the point 24 ofintersection of the inner grid straps 20 is controlled as shown in FIG.3M, wherein six pulses of the laser beam are directed onto the workpiece to form the intersect weld 32, each pulse having a pulse width of6.2 ms, a rate of 20 pulses per second (pps), an average power of 350watts, and a peak power of 2,580 watts. The intersect welds 32 areformed by turning on the laser beam 178 when the fuel rod grid 16 hasbeen disposed in an aligned position with respect to the laser beam 178.

The next step is shown in FIG. 3B, wherein the fuel rod grid 16 isrotated about its Y axis 90° by a mechanism to be explained, whereby afirst set of the slot and tab welds 34 and a first corner seam weld 30are performed. These welds are seam welds which are carried out bymoving the fuel rod grid 16 along its X axis while directing the laserbeam 178 onto the work piece. In an illustrative embodiment of thisinvention, the slot and tab welds 34 are effected with a laser beam 178of a pulse width of 2.2 ms, a pluse frequency of 50 pps, and an averageof 350 watts, with the fuel rod grid 16 being moved at a rate of 30inches per minute (IPM). FIG. 3B shows the relative position of thelaser beam 178 to effect each of the slot and tab welds 34a and thecorner seam weld 30a.

Next, as shown in FIG. 3C, the fuel rod grid 16 is rotated in aclockwise direction so that the opposing outer grid strap 22b is alignedwith respect to the laser beam 178, whereby a second set of slot and tabwelds 34b and a second corner seam weld 30b may be carried out.Thereafter, as shown in FIG. 3D, the fuel rod grid 16 is rotated 90°counter-clockwise to its original position as shown in FIG. 3A, and thefuel rod grid 16 and its weld fixture are removed from the weldingchamber.

As shown in FIGS. 3E to 3H, a similar set of welding steps are carriedout. After removal from the chamber, the fuel rod grid 16 and its weldfixture are turned over to dispose its vane side down and are rotatedabout its Z-axes 90° in clockwise direction so that the unwelded outergrid strap 22c faces the door of the welding chamber. The grid 16 andits weld fixture is locked into a secure position with respect to thewelding chamber and the laser beam. Initially, the air within thewelding chamber is purged with argon gas to an acceptable level ofpurity. Thereafter, as shown in FIG. 3E, the fuel rod grid 16 isincrementally moved through a series of steps along the X and Y axes,whereby each of the intersect welds 32 is effected as described above.After the completion of the intersect welds 32, the fuel rod grid 16 isrotated 90° in a counter-clockwise direction about its Y axis so thatits outer grid strap 22c is brought beneath the laser beam 178, wherebya third set of slot and tab welds 34c is carried out and a third cornerseam weld 30c effected. Next, as shown in FIG. 3G, the fuel rod grid 16is rotated 180° about its Y axis to present the fourth outer grid strap22d to the laser beam 178, whereby a fourth set of slot and tab welds34d, and a fourth corner seam weld 30d may be carried out. Thereafter,in the step as shown in FIG. 3H, the fuel grid 16 is rotated 90° in acounter-clockwise direction to its original position before it and itsweld fixture are removed from the welding chamber.

Referring now to FIGS. 3I to 3L, there is shown the process by which theguide sleeves 36 are welded to the fuel rod grid 16. Initially, the fuelrod grid 16 is removed from its welding fixture as required for thesteps in FIGS. 3A to 3H and placed into a sleeve welding fixture asdescribed in the copending application entitled "FUEL GRID WITH SLEEVESWELDED IN NOTCHED GRID STRAPS" (Ser. No. 414,232) the sleeve weldingfixture includes a plurality of fixture pins disposed through selectedof the cells formed by the inner grid straps 20 for receiving the guidesleeves 36, i.e. those openings having the notches 38 disposed in theperipheral edges thereof as seen in FIG. 3J. In particular, the fixturepins accurately position the guide sleeves 36 so that its axis isdisposed centrally of and parallel with the surfaces of the inner gridsteps 20. With the guide sleeves 36 accurately aligned and assembledwith respect to the fuel rod grid 16, the grid 16 and its sleeve weldingfixture are disposed into the welding chamber and affixed with respectto the chamber and to the laser beam 178. Thereafter, the air is purgedwith argon gas to the desired level of purity. Thereafter, as shown inFIG. 3J, the fuel rod grid 16 is rotated 45° in a counter clockwisedirection and thereafter the grid and sleeve welding fixture is lockedinto that position at 45° with respect to the path of the laser beam 178as shown in FIG. 3J. Thereafter, a series of notch seam welds 40 iscarried out at a pulse width of 6.2 ms, at a pulse frequency of 20 PPS,an average power of 255 watts, and at a welding speed of 10 IPM. Thefuel rod grid 16 is moved along the Y axis at the noted rate while thelaser beam 178 is pulsed. As will be explained in detail later, it isnecessary to refocus the laser beam 178 for each horizontal row of guidesleeves 36 as shown in FIG. 3J. A series of notch seam welds 40 iseffected by moving the fuel rod grid 16 along its Y axis, bringing eachguide sleeve 36 into position with respect to the laser beam 178,turning on the laser beam to effect the notch seam weld 40, andthereafter moving the fuel rod grid 16 to align the next guide sleeve36. After a single horizontal row of guide sleeves 36 has been welded,the fuel rod grid 16 is moved along its X axis to position the next rowof guide sleeves 36 in alignment with respect to the laser beam 178.Thereafter, it is necessary to refocus the laser beam 178 to effect thenotch seam welds 40. As seen in FIGS. 3J and 3K, the guide sleeve 36fits into four notches 38, and notch seam welds 40 are effected onopposing sides of the guide sleeves 36.

After one side of the guide sleeve 36 has been welded, it is necessaryto rotate the grid 16 90° in a counter-clockwise direction as shown inFIG. 3K to expose the other, opposing notch 38 to the laser beam 178.After rotation, a series of notch seam welds 40 is carried out asexplained above. Finally, in step FIG. 3L, the fuel rod grid 16 isrotated 45° in a clockwise direction to its original position before thegrid 16 and its sleeve welding fixture are removed from the weldingchamber to complete the welding steps of the fuel rod grid 16.

Referring now to FIG. 4, there is shown the laser welding system 100 forcontrolling the series of welds and in particular the intersect welds32, the slot and tab welds 34, the corner seam welds 30, and the notchseam welds 40 necessary to secure the inner and outer grid straps 20 and22 together to form the fuel rod grid 16 and to secure the guide sleeves36 to the grid 16 by controlling a laser system 102 (shown in detail inthe subsequent drawings) to emit a laser beam 178 of controlled energyto successively and precisely position the grid 16, and to control thesupply of a suitable inert gas, e.g. argon, in which to carry out thelaser welding of the aforementioned welds. Each of the work pieces, e.g.the fuel rod grids 16, is successively moved to each of the weldpositions by its positioning module 106a and 106b, the module 106a beingshown in FIG. 4. In particular, a welding chamber 108 is associated witheach of the positioning modules 106 for receiving its grid 16 toestablish an environment in which the laser welding may be carried outand, in particular, to establish an atmosphere of the inert gas whilepermitting movement of the grid 16 to effect the series of welds. Theright positioning module 106a has a right cabinet door 114a, which isshown in an open position. A like left cabinet door 114b is shown in itsclosed position and is understood to cover its corresponding leftpositioning module 106b and left welding chamber 108b. A cabinet 104further encloses the right and left positioning modules 106a and 106b,as well as the laser system 102, a main frame 122, an argon purgingsystem 118, and an argon supply system 473 to be described below. A pairof sensing mats form right and left safety zones 134a and 134b disposedimmediately in front of the right and left positioning modules 106a and106b respectively; the zones 134a and 134b sense the presence of anoperator thereon to prevent the driving of its corresponding weldingchamber 108 into a position outside of the cabinet 104.

A laser power supply 120 is shown in FIG. 4 and is coupled by suitableconductors to the laser system 102 to control the emission of coherentlight therefrom in a manner to be more fully explained below. Inaddition, there is included a computer control system 124 including afirst Computer Numerical Control (CNC) system 126a and an identicalsecond CNC 126b, associated respectively with controlling the lasingoperations as occur within the right and left positioning modules 106aand 106b. As will be explained later, the first and second CNC's 126aand 126b bid for control of the laser system 102 whereby the CNC's timeshare the control of the laser system 102. The laser power supply 120includes a laser welding display panel 132, as more fully shown in FIG.23A, and each of the CNC's 126a and 126b includes respectively itsmachine function panel (MFP) 130 as more fully shown in FIG. 23B.

The main frame 122 is more fully shown in FIG. 5 for mounting adjustablythe laser system 102 in an aligned position with respect to the rightand left positioning modules 106a and 106b. Once aligned with the lasersystem 102, the right and left positioning modules 106a and 106b arefixedly secured with respect to the main frame 122 and therefore withrespect to the laser system 102 to ensure that the alignment of thelaser beam 178 may be accurately controlled with respect to each of thepositioning modules 106a and 106b and therefore with respect to the fuelrod grids 16 carried thereby. The main frame 122 is made up of a topplate 142 and a bottom plate 43 (see FIG. 7), each welded to a frame ofsquare tubing. As shown in FIG. 7, leveling and shock pads 224 areattached to the bottom plate 143 and serve to isolate the laser system102 and the right and left positioning modules 106a and 106b fromvibrations that might be imparted to the laser welding system 100through the floor on which the laser welding system 100 rests. Inaddition, the pads 224 also dampen any vibrations that might be causedby the motor drives (to be described) associated with each of the rightand left positioning modules 106a and 106b. The top plate 142 ismachined flat after it has been welded to its frame of square tubings toprovide a reference surface for the other system components that aremounted thereon. These other components are bolted or doweled to or withrespect to the top plate 142 so that the critical alignments can bemaintained.

A kinematic support 140 is bolted to the top plate 142 and comprises apair of legs 141 and 139 each having a dowel pin securing it withrespect to the top plate 142. Each of the positioning modules 106 ismounted on the main frame 122 and comprise, as shown in FIG. 5, a baseplate 150 bolted to the top plate 142 at each of its four corners. Eachpositioning module 106 includes side walls 152 and 154, each bolted tothe base plate 150, as shown in FIGS. 5 and 10. Each module 106 includesa back wall or vertical support 248, as shown in FIG. 10. A verticalslide 252 is bolted in turn to two gussets 246, as shown in FIG. 10; anX-Y platform 244 is in turn bolted to the two gussets 246 one on eitherside, as shown in FIG. 9. As shown in FIG. 10, the X-Y platform 244receives and mounts an X-Y positioning system 288 by which its weldingchamber 108 is incrementally moved along X and Y axes under the controlof its CNC 126. Each of the positioning modules 106 further includes asshown in FIG. 5, a top or sealing plate 156 that is disposed in a closespacing (i.e. less than 0.040 inch) from, and in a substantiallyparallel relationship with, the upper flange 331 that forms the topsurface 333 of its welding chamber 108, as shown in FIG. 8. Inspectionof FIG. 8 will indicate that the critical spacing and relationshipbetween the welding chamber 108 and its sealing plate 156 permit the X-Ypositioning system 288 to move the welding chamber 108 while maintainingthe relationship of the welding chamber 108 to the sealing plate 156.This critical relationship is established by accurately positioning theplatform 244 with respect to the sealing plate 156 in a manner as willbe explained.

As shown in FIGS. 7 and 9, a pair of vertical slides 252 are fixedlysecured by dowels to the top plate 142. In particular, two dowel pinsfit through the base of each vertical slide 252 and hold it to the topplate 142. A saddle 250 is movably mounted upon each vertical slide 252and includes a positioning screw 254 extending in a substantiallyperpendicular relationship to the referenced surface of the top plate142, whereby its saddle 250, its vertical slide 252, and the platform244 may be critically aligned. The saddle 250 is bolted and doweled tothe vertical slide 252 of the positioning module 106. The saddle 250 andits positioning screw 254 are illustratively that model as manufacturedby Milwaukee Machine Components Company under their designation ModelRB16-32-20-L. The vertical slides 252 provide means for raising orlowering the X-Y platform 244 to accommodate changes in the process orin the height of the welding chamber 108. Three dowels secure the X-Yplatform 244 to its gussets 246 and may be removed therefrom to free theX-Y platform 244. Then the sets of gib keys 256 and 258 are pulled bymeans of their jacking screws. Next, a crank (not shown in the drawings)associated with the positioning screw 254 is turned to reposition up ordown the X-Y platform 244, and thereafter the vertical slide 252 isbolted and doweled to its saddle 250, and the X-Y platform 244 is boltedto the vertical slide 252. When the X-Y platform 244 is at the requiredheight, it is leveled, i.e. the spacing between the top of the weldingchamber 108 and the bottom surface of the sealing plate 156 is madeparallel with each other, and thereafter, new gib keys 256 and 258 arepositioned, and the dowel pins are refitted in new holes drilled andreamed in the side walls 152 and 154 at their new locations.

The argon purging system 118 is more fully shown in FIGS. 4, 5, and 7.Argon that spills out of the welding chambers 108 during purging andwelding falls to the bottom of each positioning module 106 to flowthrough a plurality of exhaust openings 151 within the base plates 150aand 150b. In the front of the main frame 122 are two openings 148a and148b covered by wire mesh 146. The wire mesh 146 and the main frame 122form a pair of plenums 144a and 144b which are connected by an exhaustduct (not shown) at the back of the main frame 122 via a damper 226 (seeFIG. 7), and a duct 228 to a blower assembly 230, whereby the argonspillage is forced from the cabinet 104 via an argon exhaust conduit 232from the building. The damper 226 controls the rate of argon flow. Theblower assembly 230 creates a negative pressure or vacuum when thecabinet doors 114 are closed. The blower assembly 230 may illustrativelytake the form of an exhaust blower as manufactured by Dayton ElectricCompany under their Model 2C887. The spacing between the upper surfaceof the welding chamber 108 and the sealing plate 156 as shown in FIG. 8is typically in the order of 0.030 inch to permit the movement of thewelding chamber 108 along X and Y axes, as well as to permit an evenflow of the argon from the welding chamber 108.

As shown in FIG. 5, the kinematic support 140 positions the laser system102 (and, in particular, the source of laser emission in the form of alaser rod 170 and the related optics) with respect to the referencesurface of the main frame 122 and more specifically with respect to thework pieces in the form of the fuel rod grids 16. The laser rod 170 isdisposed within a laser head housing 166 and is mounted upon an opticaltooling plate 168 which has been manufactured to very close tolerancesfor flatness. The optical tooling plate 168 is mounted upon a lasersubbase 162 which is in turn supported upon the kinematic support 140and, in particular, upon its cross beam 157 and horizontal member 159.In addition to the laser head housing 166, a movable beam switchingmirror 172 and its actuator in the form of a stepping motor 175 andstationary beam diverters in the form of mirrors 174, 176a and 176b arealso mounted upon the optical tooling plate 168. As shown in FIG. 6, thebeam switching mirror 172 takes the form of a tear drop member coupledto the stepping motor 175 to be successively rotated by the motor 175into and out of position to reflect or to transmit the laser beam 178emitted from the laser rod 170.

The laser subbase 162 supports the optical tooling plate 168 and, inturn, is mounted upon the kinematic support 140. The kinematic support140 is a weldment made of square tubing and provides the rigiditynecessary to maintain the critical alignment between the laser beam 178emitted from the laser rod 170, and the fuel rod grids 16. The lasersubbase 162 is bolted to a pair of leveling jacks 158a and 158b disposedat either end of the cross beam 157. A spherical bearing 160 is disposedon a rearward portion of the horizontal member 159 to provide a singlepoint of support for the laser subbase 162, whereby it may rotate aboutan axis 164 as each of the front leveling jacks 158a and 158b is raisedor lowered. The spherical bearing 160 is disposed at a fixed height toprovide a pivot about which the laser subbase 162 may be either liftedstraight up or tilted to the required angle by the leveling jacks 158aand 158b.

The plane of the laser subbase 162 must remain rigid while the jackingforces are applied thereto by the leveling jacks 158A and 158B duringthe initial alignment of the laser welding system 100. As will beexplained below, the laser subbase 162 also supports a pair of Z-axislaser assemblies 222 (see FIG. 7), whereby corresponding laser focusinglens assemblies 204 (see FIG. 6) may be rectilinearly adjusted to focusthe laser beam onto the fuel rod grids 16 within the correspondingwelding chambers 108. The laser subbase 162 provides a bolting surfacefor mounting the Z-axis positioning assemblies 222 (see FIG. 7). EachZ-axis laser assembly 22 is rigidly secured to the laser subbase 162 sothat it carries the laser focusing lens assembly 204 along its Z-axisthat is perpendicular to the top surface of the laser subbase 162. Asshown in FIGS. 5 and 6, the laser rod 170 emits a laser beam 177 that isfocused onto the beam switching mirror 172, which alternately directsthe laser beam 177 first to the vertical directing mirror 176a and thento the vertical directing mirror 176b, thus forming a right laser beam178a and a left laser beam 178b. The laser beams 178a and 178b aredirected through openings 180a and 180b within the positioning modules106a and 106b respectively.

The laser system 102 as shown in FIG. 5 and schematically in FIG. 6 may,in one illustrative embodiment of this invention, take the form of thatlaser system manufactured by Raytheon under their model designationnumber SS500. The laser system 102 includes the laser rod 170illustratively taking the form of a Nd:YAG crystal laser and a pair oflinear krypton flash lamps disposed in a high efficiency laser head. Thelaser head includes a total reflecting mirror 182 and a partialreflecting mirror 184 disposed on either end of the laser rod 170. Aninner cavity shutter 188 is disposed between the laser rod 170 and thetotal reflecting mirror 182 and is selectively controlled to release aselected number of lasing pulses, whereby the energy imparted to effectlaser welding may be precisely controlled in a manner to be explainedbelow. The laser head is modularly constructed to permit all opticelements thereof including the laser rod 170, the excitation lamps 186,and the mirrors 182 and 184 to be easily and independently replaced. Theexcitation lamps 186 shall be quickly replaced without disturbing theoptical alignment. Further, the excitation or flash lamps 186 are watercooled over their entire length, including their end connectors. Lamptriggering provides for parallel pulsing of the excitation lamps 186 byenergizing the cavity. The laser rod 170 shall illustratively beselected such that 400 watts average power is obtained at the work piecewith the input power to the pulse forming network not to exceed 18 KWwhen operating at pulse widths of 6 ms and 2 ms and pulse rates of 20 Hzand 50 Hz respectively. A dump shutter 190 is disposable in a firstposition to direct the laser beam 177 along a diverted path 196 into abeam absorber 194 during those periods in which the work pieces in theform of the fuel rod grids 16 are being changed within the chambers 108.An actuating mechanism 192 is shown for disposing the shutter 190 fromits first beam intercepting position to a second position, wherein thebeam 177 is focused by a beam expander lens assembly 198 to a beamdirecting mechanism comprised of the movable beam switching mirror 172and the stationary mirror 174. When the reflective mirror 171 isdisposed to intercept the laser beam 177, it is diverted along path 178ato the vertically directing mirror 176a to be directed vertically. Thelaser focusing lens assembly 204a intercepts and focuses the laser beam178a onto the fuel rod grid 16 within the chamber 108a. As shown, thelaser focusing lens assembly 204, as will be described in detail later,includes a lens 202 and a lens carrier tube 200 as rectilinearlypositioned by the Z-axis laser assembly 222. When the reflective mirror172 is rotated by the motor 175 from a position intercepting the laserbeam 177, it is diverted by the stationary reflective mirror 174 to formthe laser beam 178b as directed by the vertically directing mirror 176btowards the welding chamber 108b.

The excitation lamps 186 are energized by the power supply 120,generally shown in FIG. 4. The power supply 120 illustratively comprisesa voltage regulated DC power supply which charges a pulse formingnetwork (PFN) through a charging inductor. The related CNC 126alternately closes switches (silicon controlled rectifiers) that chargesthe PFN from the DC power supply reservoir capacitor bank and dischargesthe PFN into the excitation lamps 186 to thereby excite the laser rod170 to emit a series of laser pulses. The excitation lamps 186 shalloperate in a "simmer" mode of operation, in which the lamps 186 areoperated at a low DC current level below lasing threshold, and highcurrent pulses are superimposed on the simmer current for generatinglaser pulses. The PFN shall provide pulses of 2 ms and 6 ms.

To assist in the initial alignment of the weld chamber 108 and, inparticular the fuel rod grid 16 with respect to the laser beam 178,there is provided means for sighting the grid 16 and, in particular, todetermine its exact position with respect to laser beam 178 in the formof an alignment TV camera 206 that is aligned to establish an image path214 coinciding with the path of the laser beam 178a. As shown in FIG. 6,the image path 214 is focused by a lens 210, selectively passed by aBureau of Radiological Health (BRH) or safety shutter 212 and directedthrough the partially transmissive mirror 176 to the TV camera 206. Thelens 202, in addition to focusing the laser beam 178 onto the fuel rodgrid 16, also focuses with the assistance of lens 210 the image of thegrid 16 onto the TV camera 206. As will be explained below, the laserfocusing lens assembly 204 also includes an illuminating lamp that isselectively energized to illuminate the grid 16 for alignment purposes.The BRH shutter 212 is selectively opened and closed to permit alignmentof the grid 16 with respect to the laser beam 178, remaining closedduring all other periods as a safety measure.

As illustrated in FIG. 6, each of the welding chambers 108 may be movedfrom a first, welding position as shown in the dotted line to a second,out position. When the welding chamber 108 is in its second position,the laser beam 178 is directed by its vertically directing mirror 176onto a power measuring device or thermopile 218, as supported within ashield tube 216. As will be shown later, the shield tube 216 is mountedon a rearward portion of the welding chamber 108 and includes arestricted opening 220 whereby the laser beam 178 may be effectivelyconfined within the shield tube 216. Periodically, the welding chamber108 is disposed to its second, out position and the laser beam 178 isdirected onto the thermopile 218 to provide an indication of the poweroutput of the laser rod 170 actually impinging onto the fuel rod grid16. Under the heavy duty load imposed upon the laser system 102, it iscontemplated that the laser efficiency will attenuate due to theexhaustion of the laser rod 170 and/or its excitation lamps 186, as wellas due to the presence of smoke and debris given off during the laserwelding. Thus, in order to provide accurate, reproducible welds, thevoltage applied to the excitation lamps 186 is increased over the lifeof the laser system 102 dependent upon the thermopile measurements.

The cabinet 104 of the laser welding system 100 serves to confine theargon escaping from the welding chambers 108 so that it may be exhaustedby the argon purging system 118 as described above. In order to permitthe welding chamber 108 to be brought to its second, out position,wherein the work piece and, in particular, the fuel rod grid 16 may bereplaced, the cabinet doors 114 are mounted to be rectilinearly moved toan open position as shown in FIG. 4. In an illustrative embodiment ofthis invention, a door opening mechanism 234 is shown in FIG. 7 ascomprising two cable cylinders that are bolted to the main frame 122. Anauxiliary air cylinder keeps constant tension on the cable and takes upstretch that occurs during operation of the cabinet doors 114. The airpressure on these cylinders is controlled by a regulator. The air to thecable cylinders is controlled by a solenoid valve. The doors 114 arepermitted to move along rails mounted upon blocks. In an illustrativeembodiment of this invention, the air operated cable cylinders may takethe form of those devices as manufactured by Tolomatic under their modelnumber 100-150.

Referring now to FIGS. 8, 9 and 10, there is shown a slide table 262that permits the welding chamber 108 to be removed from the cabinet 104to its second, out position, wherein the machine operator may remove thefuel rod grid 16 from the welding chamber 108. To this end, the slidetable 262 is mounted upon the accurately positioned X-Y platform 244 tobe positively driven by a slide drive motor 266 in a rectilinear fashionbetween its first welding position and its second, out position withrespect to the cabinet 104. The slide table 262 includes a safety rail264 that protrudes in advance of the leading edge of the slide table 262to prevent operator injury. The slide drive motor 266 is coupled by adrive chain 272 to a screw drive 268 which is threadably received by ashoulder bolt 274 to drive a support bracket 276 fixedly attached to theslide table 262. As particularly shown in FIG. 10, the screw drive 268is mounted at either end upon a pair of pillow blocks 270. As more fullyshown in FIGS. 8 and 9, there are provided a pair of bearing shafts 278fixedly secured to the underneath surface of the slide table 262 andoriented substantially parallel with each other to permit the desiredrectilinear travel of the slide table 262 between its first and secondpositions. As shown in FIGS. 8, 12, and 13, each of the bearing shafts278 includes a shaft support 310 which is disposed at either end of thebearing shaft 278, is bolted to the lower surface of the slide table 262and is secured to the shaft 278 by a bolt 311. In turn, a pair of pillowblocks 282 is disposed along the length of the shaft 278 to receive andsupport it for rectilinear motion.

As shown particularly in FIG. 11, means are provided for limiting themotion of the slide table 262 between its inner and outer positions,taking the form of a stop 308 fixedly secured to the slide table 262. Oneither side of the stop 308, there are disposed stop brackets 300 and302 having positioning nuts 304 and 306 threadably received therein,respectively. The positioning nuts 304 and 306 are set to variablyselect the limits of motion of the slide table 262. The stop brackets300 and 302 are securely attached by pins to the X-Y platform 244.

Referring now to FIGS. 8 and 9, means are shown for accuratelypositioning the X-Y platform 244 and therefore the welding chamber 108in its first, welding position within its positioning module 106 and inits second, out position removed from the cabinet 104, wherein theoperator may readily remove the fuel rod grid 16 from the weldingchamber 108. It is critical that the welding chamber 108 and, inparticular, its fuel rod grid 16 be disposed accurately with respect tothe laser beam 178 as shown in FIGS. 6, 8 and 9. To this end, a frontlocator assembly 284 selectively directs its locator pin 316 as shown inFIG. 12 from a first, withdrawn position, to a second, locking position,wherein it is disposed within an opening 318 of a positioning member 317fixedly attached to the slide table 262 to thereby precisely positionthe slide table 262 with respect to the laser beam 178. A similarpositioning member 312 is disposed at a rearward portion of and isfixedly attached to the slide table 262 to engage the locator pin 316 ofthe front locator assembly 284, to thereby position and hold the slidetable 262 and therefore the welding chamber 108 in its second, outposition. As more specifically shown in FIG. 12, the front locatorassembly 284 includes a locator bracket 322 fixedly attached to theplatform 244 at one end and having at its other end a crangor bracket320 from which is suspended by a clevis 324, an actuator 314 for drivingthe locator pin 316. A second or back locator assembly 286 is shown inFIGS. 7 and 9 and serves to fixedly secure the slide table 262 withrespect to the laser beam 178. The back locator assembly 286 is fixedlysecured to its positioning module 106 by a locator bracket 323 attachedto the vertical support 248 and includes an actuator 315 and a locatorpin 319 driven thereby from a first, withdrawn position to a second,locking position, whereby the locator pin 319 engages an opening 325 ofa positioning member 321 affixed to the slide table 262. In this manner,the slide table 262 is affixed at diagonally opposed corners by thelocator pins 319 and 316 of the back and front locator assemblies 286and 284, respectively, thus ensuring a fixed relationship between thetable slide 262 and the laser beam 178. The front and back locatorassemblies 284 and 286 may illustratively take the form of plungermechanisms as manufactured by DeStaco.

Referring now to FIGS. 8 and 10, each of the positioning modules 106includes means in the form of an X-Y positioning system 288 forprecisely positioning the welding chamber 108 and, in particular, thefuel rod grid 16 contained therein in a plurality of preciselycontrolled positions along X and Y axes of a plane, as well as to rotatethat plane at a precisely controlled angle about the Y axis, whereby avariety of welds may be effected by the laser beam 178. The X-Ypositioning system 288 is disposed as shown in FIG. 10 as being mountedupon the slide table 262 for supporting and positioning the weldingchamber 108. The X-Y positioning system 288 includes an X positioningtable 290 and a Y positioning table 292 mounted thereon. The X and Ypositioning tables 290 and 292 may illustratively take the form of thatmechanism as manufactured by the Shaum Manufacturing Company under theirproduct number DC1212. The X positioning table 290 serves to move thechamber 108 in a direction substantially perpendicular to the plane ofFIG. 8, whereas the Y positioning table 292 moves the chamber 108 alonga direction perpendicular to the plane of FIG. 10. The Y positioningtable 292 is associated with a Y drive motor 296 that includes aresolver and tachometer, whereby precise incremental distances may beimparted to the welding chamber 108. Similarly, the X positioning table290 is associated with an X drive motor, resolver, and tachometer 294.

As will be explained later in detail, a B axis rotation drive 238 asgenerally shown in FIG. 9 is engageable with the welding chamber 108 andin particular with a rotatable fixture assembly 240 as rotatably mountedwithin a side wall of the welding chamber 108 to rotatably position arotatable fixture 242 as shown in FIG. 9. It is understood that the fuelrod grid 16 is attachable to the rotatable fixture assembly 240, wherebyit may be rotatably disposed about the Y axis.

The welding chamber 108 and its rotatable fixture assembly 240 will nowbe described more specifically with respect to FIGS. 14 and 15, ascomprising a bottom plate 326, front and back walls 329a and 329b, andside walls 327a and 327b. An upper flange 331 is disposed around theupper periphery of the aforementioned walls to provide a machined, flatseal surface 333 that is disposed in a close, precisely parallelrelationship with the lower surface of the sealing plate 156. Thisprecise relationship between the seal surface 333 and the sealing plate156 permits the even flow of the argon from the welding chamber 108 intothe positioning module 106, as well as the movement of the weldingchamber 108 and its fuel rod grid 16 along the X, Y axes of a planesubstantially parallel to the lower surface of the sealing plate 156.

As shown in FIG. 15, a support gasket 332 is disposed on the bottomplate 326 for forming a plenum chamber for receiving the flow of argonthrough an argon input port 338. The plenum chamber is formed by abottom cover 328, a diffuser plate 330, and a hold down strap 334 whichis configured as a frame and is disposed to retain the peripheral edgeof the diffuser plate 330 with respect to the support gasket 332. A pairof manifold tubes 336, as shown in FIGS. 14 and 15 (only one beingillustrated), distributes the flow of argon within the plenum chamber.Significantly, the diffuser plate 330 is made of a uniformly sintered,stainless steel of approximately 60% density and in an illustrativeembodiment of this invention is made of a material known as "Feltmetal"having dimensions of 1/8 inch thickness and 15 inches square andmanufactured by Brunswick under their number FM1110. The diffuser plate330 covers the entire bottom of the welding chamber 108 and providesmeans for producing a laminar gas flow which "floats" the air out of thewelding chamber 108 with a minimum of turbulence. The higher density ofargon is evenly distributed over the cross sectional area of the weldingchamber 108 to effectively exclude air from the welding chamber 108,whereby an atmosphere of inert gas, e.g. argon, may be established witha high degree of purity. It has been found that an atmosphere of apurity in the order of 10 parts per million (PPM) water and of 7 PPMoxygen will produce significantly improved welds of the materialZircaloy. Various porous metal products were tried to identify the mosteffective material; it was ascertained that improved results wereobtained with a thicker, higher density material, e.g. a sintered,stainless steel fiber plate having a density of 60%. Further, it issignificant that the diffuser plate 330 covers substantially the entirebottom of the welding chamber 108, with as little non-diffusingsupporting structure as possible. As the diffuser area decreases inrelationship to the chamber bottom surface area, the time and quantityof argon required to purge the welding chamber 108 of air and moistureincreases. For example, a diffuser plate 330 that would cover only 1/4of the bottom surface is no more effective than simply directing a flowof gas into the welding chamber 108 through a tube or other jet. Asillustrated in FIG. 15, the diffuser plate 330 is effectively sealed tothe side walls 327 and the front and back walls 329 so that the argonflowing into the plenum is forced to diffuse through the plate 330, andnot simply to bypass the diffuser plate 330 and to flow up along theside, front, and back walls. The illustrative structure for supportingthe periphery of the diffuser plate 330 ensures that the argonintroduced at relatively high gas flow will not deflect the diffuserplate 330. The pair of manifold tubes 336 as well as the configurationof the plenum formed by the bottom cover 328 and the diffuser plate 330ensure an even gas distribution across the cross section of the weldingchamber 108. As mentioned above, the seal surface 333 is disposed at asubstantially uniform, parallel spacing with respect to the lowersurface of the sealing plate 156 of a distance less than 0.040 inch andin one illustrative embodiment at a spacing of 0.030 inch to provide aneven distribution into and from the welding chamber 108. The use of aseal between the welding chamber 108 and the sealing plate 156 wasavoided in that it tended to impose an unnecessary drag on the X-Ypositioning system 288, thereby slowing the rate at which the weldscould be made. It is understood that a flow of gas, as will be discussedin detail later, into and from the welding chamber 108 prevents othercontaminating gases from flowing into the chamber 108. As a result ofmaintaining an even flow of the inert gas into the welding chamber 108,the purity of the welding atmosphere within the welding chamber 108 isensured. As discussed above, weld contamination is prevented to a highdegree sufficient to ensure the structural integrity of the fuel rodgrid 16 even when exposed to the hostile environment of a nuclearreactor, wherein the fuel rod grid 16 is subject to high flows ofsuper-heated water tending to rapidly contaminate any weld and leadingto the structural deterioration of the grid 16 and the rupturing of thefuel rods 18.

The welding chamber 108 receives and rotatively supports the rotatablefixture 242 upon which the fuel rod grid 16 is mounted for laser weldingwithin the inert atmosphere. As shown in FIG. 14, the rotatable fixture242 includes a first shaft 510 and a second, fixture shaft 368. Thefirst shaft 510 is rotatively received by a bearing 346 as mounted by aport cover 342 within a port 343 of the side wall 327b of the weldingchamber 108. A feed cover 348 is in turn mounted to cover the bearing346 and to support and seal an argon input port 500 by which argon isintroduced by a flexible hose 490, to the rotatable fixture 242. Theshaft 368 is mounted within a bearing 356 (see FIG. 15) as mountedwithin a bearing housing 344 attached to the side wall 327a. In turn,the shaft 368 is fixedly coupled to a positioning wheel 358, which incontrollably rotated to selectively rotate and fixedly orient theposition of the rotatable fixture 242 within the welding chamber 108with respect to the laser beam 178. A locating mechanism 370 is mountedby a housing 372 attached to the side wall 327a for positively lockingthe position of the positioning wheel 358 and therefore the angularposition of the rotatable fixture 242, and for releasing the positioningwheel 358, whereby it may be rotated by the B axis rotation drive 238 aswill be explained. The locating mechanism 370 includes a locator pin 378that is biased by a spring 376 into one of the openings 379 topositively position and lock the positioning wheel 358 in that position.The locating mechanism 370 also includes a position shaft 374 attachedto the pin 378 and disposed within the housing 372 for axially guidingthe pin 378 and a release bracket 380 that is engageable to depress thespring 376 and to release the positioning wheel 358 for rotation by theB axis rotation drive 238.

As shown in FIGS. 14, 18, 19, and 20, the rotatable fixture 242comprises a frame 502 disposed between the shafts 510 and 368 that arealigned on either side of the frame 502 with respect to each other. Apair of struts 504 extend parallel of each other between opposing sidesof the frame 502 to form openings 505 through which the laminar flow ofargon is directed to the fuel rod grid 16 as supported on a top mostsupport surface 540. The fuel rod grid 16 is held within a weldingfixture 542 which is in turn locked to the rotatable fixture 242 by apair of locating pins 524. The welding fixture 542 is shown in phantomline in FIG. 15 and is described in copending application entitled"WELDING PLATES FOR A FUEL ROD GRID" (Ser. No. 414,265). Argon isintroduced into the welding chamber 108 through the first argon inputport 338 at the bottom-most portion of the welding chamber 108 and alsothrough the second argon port 500 to be directed through a traverseconduit 512 and therefrom through a pair of axial conduits 514 to bedischarged through output ports 506 within the struts 504. A seconddiffuser plate 520 is mounted to cover the openings 505 and is securedto the rotatable fixture 242 by a retaining frame 518 that is held byscrews to retain the second diffuser plate 520 within a recess 516formed within the rotatable fixture 242. Thus, there is shown means forproviding a further flow of the inert gas argon through the work pieceand, in particular, through the inner and outer grid straps 20 and 22 ofthe fuel rod grid 16, thereby ensuring the atmosphere purity and theintegrity of the laser welds made therein.

The locating pins 524 comprise, as shown in FIGS. 19 and 21, a lockinghead 526 whose lower edge securely locks the welding fixture 542 inplace upon the support surface 540 of the rotatable fixture 242. Thelocking head 526 is pivotally and flexibly mounted by a cantilevermember 528 secured at its other end to a mounting member 530. In turn,the mounting member 530 is disposed within an opening 534 in such amanner that its collar 532 is securely fit within a recess 536 andretained therein by a screw 538. In this manner, the fuel rod grid 16 assupported by its welding fixture 542 may be gradually lowered onto thesupport surface 540, so that openings within the welding fixture 542 arealigned with and received by the locking heads 526 of the locating pins524, which being spring biased are deflected to be directed through suchopenings and thereafter, the heads 526 are biased by their cantilevermembers 528 into a support locking position.

With reference to FIGS. 15 and 16, the detailed structure of thelocating mechanism 370 will now be explained. The shaft 510 of therotatable fixture 242 is rotatably mounted by a bearing 346, which isheld within the thrust housing 340 by a locknut 350. In addition, a pairof hand knobs 352 is mounted upon the port cover 342 to assist in themovement of the welding chamber 108. The other shaft 368 of therotatable fixture 242 is rotatably supported by the bearing 356 assupported within the bearing housing 344 and is retained therein by ahold down cover 354 threadably secured to the side wall 327a. A couplingcollar 364 has an opening for receiving the end of the shaft 368 and isrotatively coupled to the positioning wheel 358 by a dowel pin 366disposed through the collar opening and a slot within the shaft 368. Atoothed coupling member 362 is fixedly attached to the positioning wheel358 and has teeth of a configuration and spacing to engage the teeth ofa corresponding toothed coupling member 384 as selectively driven by theB axis rotation drive 238 to impart a rotation to the positioning wheel358 and to its rotatable fixture 242.

Referring to FIG. 16, there is shown a plurality of sensor strips 382a,382b, 382c, 382d and 382e disposed radially of the positioning wheel 358and serving to provide an indication of the angular position of thepositioning wheel 358 and its rotatable fixture 242 about the B axisposition, indicated by the arrow in FIG. 16, i.e. the angular positionabout the Y-axis as disposed perpendicularly with respect to the planeof FIG. 16. As seen in FIGS. 14 and 15, the sensor strips 382 havedifferent configurations, so as to distinctly actuate corresponding onesof a plurality of proximity switches 402, 402a, 402b, 402c, and 402d,dependent upon the B axis position of the positioning wheel 358. Forexample, the sensor strip 382a actuates only switch 402a to provide abinary signal "100" to provide an indication of the B axis position ofminus 90°; strip 382b actuates only the proximity switch 402b to providea binary signal "010" to provide an indication of a minus 45° angularposition; sensor strip 382c actuates the binary proximity switches 402aand b, to provide a binary signal "110" indicative of a zero angularposition; sensor strip 382d actuates switch 402c to provide a binarysignal "001" to provide an indication of a plus 45° position; and sensorstrip 382e actuates switches 402a and 402c to provide a binary signal"101" to provide an indication of an angular position of plus 90°. Thethree proximity switches 402a, b, and c are actuated by the sensorstrips 382 to provide binary signals indicative of the B axis positionof the rotatable fixture 242, whereas the lower most or last proximityswitch 402d is actuated to indicate that a sensor strip 382 is in analigned relation to the proximity switches 402, which provide a binarysignal indicative of the B axis angular position of the positioningwheel 358.

The welding chamber 108 is moved by the Y positioning table 292 to theleft as seen in FIG. 15, whereby the sensor strips 382 engageselectively as indicated above the proximity switches 402 to providetheir indication of the B axis angular position of the positioning wheel358. As will be explained below, the X and Y positioning tables 290 and292 are under the control of its CNC 126, whereby, when it is desired torotate the positioning wheel 358 and its rotatable fixture 242, the Ypositioning table 292 is actuated to move the welding chamber 108 to theleft as seen in FIG. 15, whereby the rotatable fixture assembly 240engages the B axis rotation drive 238 and, in particular, its toothedcoupling member 362 engages the toothed coupling member 384 to effect arotative coupling of the positioning wheel 358 to the rotatable fixture242. A spring 394 serves to bias the coupling member 384 to mate withthe coupling member 362. A fifth proximity switch 402e is also mountedupon the motor bracket 400 and is activated as the release bracket 380is pressed there against as the welding chamber 108 is moved intoposition where the coupling members 362 and 384 engage each other. Theactivating or closing of the proximity switch 402e in turn energizes thesolenoid 406, whereby the trip 404 is disposed to the left as shown inFIG. 15, against the biasing action of a spring 408, thus engaging therelease bracket 380, depressing the spring 376 and removing the locatorpin 378 from one of the openings 379 within the positioning wheel 358. Amotor 388 is now energized to drive the coupled toothed coupling members362 and 384 to impart a rotational movement to the rotatable fixture242. The motor 288 continues to rotate the positioning wheel 358 untilit reaches its new position as sensed by the proximity switches 402.Upon reaching its new position, the solenoid 406 is deenergized, and thelocator pin 378 reinserted into that locator opening 379 associated withthe new wheel position.

As shown in FIGS. 14 and 16, the welding chamber 108 includes a moisturesensor 410 for providing an indication of the moisture content withinthe chamber atmosphere in terms of parts per million. In addition, theshield tube 216 is mounted by a shield bracket (not shown) on the backwall 329b of the welding chamber 108. A meter bracket 412 is alsodisposed on the back wall 329b for mounting the thermopile 218 inalignment with the laser beam 178 when the welding chamber 108 isdisposed to its second, out position as shown in dotted line in FIG. 6and the lens carrier tube 200 is disposed in axial alignment with theshield tube 216. As noted above, the laser system 102 is periodicallycalibrated to ensure that precise quantities of laser energy areimparted by the laser beam 178 to the fuel rod grid 16. Further, asshown in FIG. 16, the sealing plate 156 includes an opening 426 that isdisposed in alignment with the work piece, e.g. the grid 16, when thewelding chamber 108 is disposed and locked in its first, weldingposition by the front and back locator assemblies 284 and 286. When thelaser focusing lens assembly 204 is aligned with the work piece asmounted upon the rotatable fixture 242, the Z-axis laser assembly 222 isactuated to direct the laser focusing lens assembly 204 along the Z-axisdownwardly as shown in FIGS. 6 and 15, whereby the assembly 204 and inparticular its lens 202 are positioned to focus the laser beam 178 uponthe work piece. In that position, the laser focusing lens assembly 204is axially aligned within a shielding ring 420 disposed concentricallyabout the opening 426. In addition, a shielding cap 422 is mounted uponthe ring 420 and includes an inwardly directed flange to form an opening424 of but slightly greater dimension than that of the laser focusinglens assembly 204, thereby preventing operator exposure to the laseremission directed into the welding chamber 108.

Referring now to FIG. 17, there is shown in detail the laser focusinglens assembly 204 as mounted upon the Z-axis laser assembly 222 formovement rectilinearly along the Z-axis to and from the welding chamber108 and in particular, its shielding cap 422. The laser focusing lensassembly 204 is described in copending application entitled "LASER LENSAND LIGHT ASSEMBLY" (Ser. No. 414,205). The assembly 204 includes thelens carrier tube 200 disposed vertically and aligned concentricallywith respect to its laser beam 178. A tube base 430 is disposed at thebottom of and connected to the lens carrier tube 200 for removablyreceiving a lens mounting member 440. As shown in the detailed drawingof FIG. 19, the mounting member 440 is configured to receive a lockingring 436 having a plurality of helical grooves 434, e.g. 3, disposed toreceive a corresponding plurality of lock pins 432. Upon rotation, thelocking ring 436 and its mounting member 440 may be locked to the tubebase 430. A safety hood 438 is made of conical configuration to directthe focused laser beam 178 onto the work piece and has a threadedperipheral portion for engaging a set of threads upon the innerperipheral portion of the mounting member 440. In similar fashion, thelens 202 is supported within a central opening of a mounting member 440and is retained therein by a retaining ring 442 being peripherallythreaded to engage a set of threads upon the mounting member 440,whereby the retaining ring 442 may be screwed onto the mounting member440 to retain the focusing lens 202.

The safety hood 438 is disposed through an opening within a lamp hood446 to affix the hood 446 to the mounting member 440. A pair ofquartz-halogen lamps 428 is disposed within the lamp hood 446 toilluminate the work piece thus permitting alignment of the work piecewith respect to the Z-axis or laser beam 178. The operating temperatureof the lamps 428 prevents welding debris from collecting thereon. Aswill be described in detail below, the operator aligns the work piecewith respect to the laser beam 178 by viewing the CRT 133 displaying theimage as taken by the TV camera 206 whereby the operator may place itselectronic rectical on an initial weld site to determine an offsetbetween a home position and the sighted, first weld site; this offset isthen automatically incorporated into the control signals as applied tothe X-Y positioning system 288, whereby each of the welds is preciselypositioned with respect to the laser beam 178. The lamp 428 is energizedby wires directed to the lens hood 446 via an electrical input port 449as inserted into a tube mounting portion 201 and a conduit 451 leadingfrom the portion 201 to the tube base 430. In similar fashion, a flow ofthe inert gas, e.g. argon, is introduced into the space formed by thesafety hood 438 and the lens 202 by an argon input port 448 threadablyattached within an opening of the tube mounting portion 201 andtherefrom via a conduit 450 leading to the mounting member 440. Themounting member 440 has an argon jet formed in alignment with theconduit 450, whereby the flow of argon is formed into a jet directedinto the hood 438, whereby debris or contaminants as emitted during thewelding process will be effectively removed so as not to attenuate thelaser beam 178 focused onto the work piece. As seen in FIG. 17, the flowof argon escapes from the safety hood 438 into the space confined by theshielding ring 420 and the cap 422 to be exhausted via an output port454.

The lens carrier tube 200 and in particular the tube mounting portion201 is mounted by a mounting assembly 460 on the Z-axis laser assembly222. A bellows 456 is secured to the topmost portion of the lens carriertube 200 to provide shielding of the laser beam directed therethrough,while permitting the Z-axis laser assembly 222 to move the laser lensassembly 204 rectilinearly along the path of the laser beam 178 as shownin dotted line in FIG. 17. In this fashion, the lens 202 may be variablydisposed along the Z-axis to permit precise focusing of the laser beam178 onto the work piece whose position may be changed with respect tolaser beam 178, as by rotating the rotatable fixture 242 to effectdifferent types of welds.

As shown in FIGS. 7 and 8, the expandable bellows 456 is connected tothe topmost portion of the lens carrier tube 200 and to a protectivehousing 461 by a bellows adapter 464. The Z-axis laser assembly 222includes a Z-axis table 458 upon which the laser focusing lens assembly204 is mounted by the lens mounting assembly 460, and is incrementally,selectively driven by a Z-axis drive motor 470 as shown in FIG. 7. In amanner similar to the X and Y drive motors 294 and 296, the Z-axis drivemotor 470 also includes a resolver and a tachometer to provide outputsignals indicative of the precise position of the Z-axis table 458, aswell as its speed of movement. The Z-axis table 458 is mounted in avertical position, thereby imposing a force upon the Z-axis drive motor470 which is counter-balanced by a pair of spring powered reels 466 thatare coupled respectively by cables 472 disposed about the reels 466 andsecured to the Z-axis table 458 by a suitable securing means such as ascrew 468. The Z-axis table 458 may in one illustrative embodiment ofthis invention take the form of a table as manufactured by DesignComponents, Inc., under their designation SA100. The coupling betweenthe Z-axis drive motor 470 and the Z-axis table 458 may illustrativelytake the form of those components manufactured by Shaum Manufacturing,Inc., under their designations "Heli-Cal" Nos. 3477-16-8 and 5085-8-8.The Z-axis drive motor 470 may illustratively take the form of that DCservo controller as manufactured by Control Systems Research, Inc.,under their designation SM706RH.

An argon supply system 473 is shown in FIG. 21 for providing a flow of asuitable inert gas, e.g. argon, to the welding chamber 108 and to thelaser lens assembly 204 at selected variable rates. The laser welding ofvolatile materials such as Zircaloy, of which the inner and outer gridstraps 20 and 22 are made, must be conducted in an inert atmosphere dueto the highly reactive nature of Zircaloy to oxygen, nitrogen, andwater. Welding tests have demonstrated that an inert gas flow around theimmediate weld area of a work piece does not provide adequate shieldingfrom oxygen and water to produce the desired high quality of welds thatwill withstand the hostile environment of a nuclear reactor withoutfailure. The argon supply system 473 as shown in FIG. 21 includes thewelding chamber 108 as shown more fully in FIG. 14, as well as the laserfocusing lens assembly 204 as particularly shown in FIG. 17. The argonsupply system 473 comprises an argon supply tank 474 that is coupled toa flow valve 476 which separates the argon supply tank 474 from theremainder of the system 473. This valve 476 is kept fully open exceptwhen it is necessary to shut down the entire system. The argon flowsfrom the tank 474 through the valve 476 to a regulator 478, whichestablishes the system pressure so as not to exceed a maximum level,e.g. 50 psi. It is contemplated that the flow of argon to each of thewelding chambers 108a and 108b and the laser focusing lens assembly 204will be controlled at a plurality of different rates depending uponwhether the grid 16 is being loaded into the chamber 16, the chamber 108is being purged, or a welding operation is occurring. For example, thepurging of the welding chamber 108 requires a relatively high flow rateof the inert gas at which time pressure should not exceed the maximumlevel. To this end, a relief valve 482 is coupled to a manifold 480 forreceiving the gas flow and for distributing it to each of a plurality ofmass flow controllers 484, 486, and 488. The mass flow controllers 484,486, and 488 are respectively connected to the welding chamber 108, tothe rotatable fixture 242 and to the laser lens assembly 204. Inparticular, a controlled rate of gas flow is provided from the mass flowcontroller 484 via a flexible hose 490 to the argon input port 338,whereby argon is directed to each of the manifold tubes 336 as shown inFIG. 15. In similar fashion, the gas flow from the mass flow controller486 is directed through the flexible hose 490 to the argon input port500 as shown in FIGS. 15 and 18, whereby argon is directed via theconduits 512 and 514 to be discharged through output ports 506 of therotatable fixture 242. It is understood that the flexible hoses 490 areprovided to permit movement of the welding chamber 108 as it is movedinto and out of the cabinet 104 by the slide table 262. The flow of gasis directed from the mass flow controller 488 via a flexible hose 490 tothe laser lens assembly 204 and in particular to the argon input port448, whereby argon may be directed via the conduit 450 and a pluralityof the jets 452 into that space immediately below the focusing lens 202.This argon flow prevents the submicron oxides produced by the laserwelding within the welding chamber 108 from contaminating the lens 202.

The moisture (H₂ O) sensor 410 is disposed within the welding chamber108 and is coupled with a moisture monitor 492. The operator and the CNC126 check the level of moisture within the welding chamber 108 duringthe purging and welding operations, whereby laser welding may beprohibited if the moisture content is greater than a specified level,e.g. 10 ppm. In addition, an oxygen probe 496 is disposed in the sealingplate 156 for sampling the argon drawn through the peripheral openingbetween the upper flange 331 of the welding chamber 108 and the sealingplate 156. It is understood that the output of the oxygen probe 496 alsoserves to provide an indication of the nitrogen content of the air inthe chamber 108. The monitoring of the atmosphere in the welding chamber108 is begun when the welding chamber 108 is disposed to its first,welding position. Each such oxygen probe or monitor 496 includes acalibrating gas inlet so there is a direct flow of gas to the probe 496.The output of the probe 496 is coupled to an oxygen analyzer 494 whoseoutput in parts per million (ppm) may be displayed upon the monitormeter 498. The CNC 126 may be programmed as will be explained so thatthe welding sequence will not be initiated until the oxygen level isbelow a programmed value, e.g. 7 ppm. During welding, the sampling ofoxygen is automatically discontinued to avoid contamination of the probe496 with welding debris.

The argon supply system 473 provides a flow of the inert gas, e.g.argon, at a substantially constant flow rate into the welding chamber108 to maintain the atmosphere within the chamber substantially pure,i.e. below the limits of oxygen and water contamination as definedabove. The flow rate is dependent upon whether the laser welding system100 and in particular its welding chamber 108 is in its loading andunloading cycle, in its purging cycle, or in its welding cycle. As willbe explained, the CNC 126 associated with the welding chamber 108directly controls the mass flow of controllers 484, 486, and 488 to anyone of a plurality of flow rates. In particular, there are fourpotentiometers for each mass flow controller. The CNC 126 actuates aselected potentiometer to provide the gas flow rate required for each ofthe loading and unloading, purging and welding cycles. To change theprogram flow rate, the CNC 126 addresses the potentiometer whereby theoperator may then adjust the potentiometer to provide the desired flowrate. The flow will appear on a suitable digital display of thecontroller. The mass flow controllers are calibrated in standard litersper minute (SLPM).

When opening the welding chamber 108 to load and unload a grid 16, thewelding chamber 108 should be slid as upon the slide 262 table withrespect to the sealing plate 156, rather than swinging the sealing plate156 open like a door. This sliding technique reduces air/argonturbulence and minimizes the air currents that would otherwise tend tomix the air with the argon in the welding chamber 108. During theload/unloading cycle, argon flow is set at a low rate to maintain theargon atmosphere as pure as possible, typically in the order of 30 CFH.A high flow rate during the load/unload cycle would cause turbulencethat would draw air into the welding chamber 108. The loading/unloadingof the grid 16 should be accomplished with a mechanical gripping deviceas described in copending application entitled "WORKPIECE GRIPPING ANDMANIPULATING APPARATUS FOR LASER WELDING SYSTEMS AND THE LIKE" (Ser. No.414,262). If such a gripping device were not employed, the operatorwould place his hands onto the chamber 108 thereby increasing theair/argon mixing and introducing additional undesirable moisture intothe argon atmosphere.

Immediately prior to the welding cycle and after the welding chamber 108has been returned to its first, welding position, i.e. beneath thesealing plate 156, the mass flow controllers 484 and 486 are controlledby their CNC 126 to effect a relatively high flow rate of the inert gasin the order of 400 CFH, whereby a welding chamber 108, as shown in FIG.16 and having approximately square dimensions of 14 by 16 by 16 inches,may be purged to lower the oxygen level to below 10 ppm in approximatelyone minute.

After the purging cycle has been completed, the laser welding system 100and in particular its CNC 126 is prepared to initiate the laser weldingcycle, during which a substantially lowered gas flow rate as controlledby the mass flow controllers 484 and 486 may be introduced into thewelding chamber 108. Also, the weld gas sampling pumps for the oxygenprobe 496 are automatically shut off to prevent contamination withwelding debris. A relatively low flow rate in the order of 30 CFH hasbeen found sufficient to maintain the welding chamber atmosphere belowthe levels of the purity defined above. As shown in FIGS. 14 and 15, theargon gas is introduced by the manifold tubes 336 and flows through thediffuser plate 330 to produce a laminar gas flow which "floats" the airout of the welding chamber 108. The higher density of argon and thesubstantially constant flow rate thereof effectively excludes air fromthe welding chamber 108. The diffuser plate 330 is made of sinteredstainless steel fibers with a plate density of approximately 60% and athickness of 0.125 inch. Further, the diffuser plate 330 coverssubstantially the entire bottom cross section of the welding chamber108, with as little non-diffusing supporting structure as possible. Asthe diffuser area decreases in relation to the cross sectional area ofthe chamber 108, the time and quantity of argon required to purge thewelding chamber 108 of air increases; this is an important considerationwhen the rapid, high production of grids 16 must be effected. Further,the diffuser plate 330 must be adequately sealed to the sides of thewelding chamber 108 so that the incoming argon is forced to diffusethrough the plate 330 and not simply bypass the diffuser plate 330 andstream up along the walls 327 and 329. The hold down strip 334 isdisposed about the upper peripheral surface of the diffuser plate 330 toprevent it from deflecting under high rates of gas flow that wouldotherwise tend to deflect the plate 330. Multiple gas inlets in the formof the pair of manifold tubes 336 improve the gas distribution into thewelding chamber 108.

In like fashion, the laser lens assembly 204 as shown in FIG. 17 neednot be tightly sealed with respect to the cap 422. The gap therebetweenprovides an opening for the argon gas to escape from the welding chamber108, when a high argon flow is used to purge the welding chamber 108 ofair. Since all gases diffuse within each other, a constant flow of gasis especially required during the welding and purging cycles to maintaina pure atmosphere. Although a small gap is preferably required betweenthe chamber 108 and the sealing plate 156, as well as between the cap422 and the laser lens assembly 204, the rest of the welding chamber 108must be free of all leaks. While argon is heavier than air and wouldtend flow out through any such leaks in the chamber 108, air also canasperate into the chamber 108 through the same hole, thus contaminatingthe atmosphere of the welding chamber 108.

Referring now to FIGS. 22A and 22B, there is shown a functional blockdiagram of the computer control system 124 and in particular of thefirst computer numerical control (CNC) 126a and the manner of itsconnection to the other CNC 126b represented only by a single blockwithin the diagram. In this regard, it is understood that the other CNC126b comprises the same elements as does CNC 126a, as shown in FIGS. 22Aand 22B. The CNC 126a comprises a central processor unit (CPU) andmemory identified by the numeral 560. In an illustrative embodiment ofthis invention, the CNC 126 and in particular its CPU 560 may take theform of that computer as manufactured by the assignee of this inventionunder their model number 2560. The CPU 560 is provided with 64K of corememory and is particularly adapted in its architecture and in itsprogramming for machine control. It is understood that a standard 2560CNC contains a basic supervisory software termed herein as either a maintask loop system or operating program, which acts in the nature of anexecutive program to oversee the operation of the entire system. In thedata structure as established within the model 2560 CNC, sets of codes,i.e. M, S, and T codes, are used to effect special or customizedoperations for which the 2560 CNC is readily adapted. In particular, apart program is programmed with the M, S, and T codes which call or bidsubroutines termed herein application subroutines, whereby selectedfunctions including the control of the argon flow and of selecting aparticular welding mode are carried out. Further, the part program isprogrammed also with X, Y, and Z codes that control the movementimparted by the X and Y drive motors 294 and 296 to its work piece, andthe Z drive motor 470 to the laser lens assembly 204, respectively. Inparticular, the X and Y codes designate the amount of movement ordestination to which the work piece in the form of the fuel rod grid 16is to be moved between welding steps. In like fashion, the Z codecontrols the amount of movement to be imparted to the laser lensassembly 204, whereby the laser beam 178 is focused upon the fuel rodgrid 16. In particular, the Z codes are needed to effect the notch seamwelds 40, wherein the rotatable fixture 242 is rotated out of its normalplane perpendicular to the laser beam 178 thereby necessitating therefocusing of the laser lens assembly 204. In addition, the memory ofthe CPU 560 has a special storage area known as the Part Program StorageArea, which is used to store the part program for execution by theoperating system program. As will be explained, the part programbasically designates the steps of the process of welding in acontrolled, inert atmosphere and more specifically, is programmed withthe M, S and T codes, whereby the mode of welding and the rate of argonflow are effectively controlled. The Parts Program Storage Area storesthe part program as described below in FIGS. 24A and 24B and theapplication routines are described in FIGS. 25A to 25R. The part programis entered into the memory of the CPU 560 by a magnetic tape drive 586via interface 590; in an illustrative embodiment of this invention, themagnetic tape drive 586 may take the form of that drive as manufacturedby Qantex under its number 220. Alternatively, the part program can bestored on a paper tape and entered via a paper tape reader 584 via amicro-processor interface 588; illustratively, the paper tape reader 584may take the form of that reader as manufactured by Decitex.Additionally, the microprocessor interface 588 also permits display ofdata messages upon the CRT 133. In addition, various parameters may beentered into the memory of the CPU 560 by the operator upon analpha-numeric key board 131 via the interface 588. The alpha-numeric keyboard 131 and the CRT 133 are mounted on the computer housings 129a and129b as shown in FIG. 4.

As shown in FIGS. 22A and 22B, the CPU 560 is associated through aplurality of closed loop axis drive and control boards 566, 568 and 570associated respectively with the X and Y drive motors 294 and 296, andwith the Z axis drive motor 470. It is understood that each of the drivemotors is associated with its tachomoter and resolver to provide anindication of the rate of travel, as well as the distance of travel,whereby extremely accurate control of the movement of the X, Y, and Ztables 290, 292 and 458 may be effected. Further, the control outputsignal as derived from the control board 566 is applied to a servoamplifier 567 to be compared with a signal indicative of motor speed, toprovide an output signal to actuate the X drive motor 294. As shownschematically, each of the motors 294, 296, and 470 is associated with alead screw 295, 297, and 471 that effects drive of its corresponding X,Y and Z tables 290, 292 and 458. A set of limit switches 572 areassociated with the lead screw 295 to sense the position of the leadscrew 295 and, therefore, its X positioning table 290 and to providesignals via an input and output interface 562 to the CPU 560. Inparticular, the limit switches 572a and 572c provide output signalsindicative that the X positioning table 290 has been disposed to itsforward and rearward most limits of travel, whereas the limit switch572b indicates that the X positioning table 290 is disposed at its homeor reference position with respect to the laser beam 178. A similar setof limit switches is associated with the lead screw 471 driving the Zaxis table 458. A set of limit switches 574a, b, and c is provided withthe lead screw 297 driving the Y table 292; a fourth limit switch 574dis associated with the lead screw 297 for detecting when the Ypositioning table 292 has been disposed in its center position, i.e.that position at which the welding chamber 108 may be removed from thecabinet 104.

As seen in FIGS. 22A and 22B, a host of peripheral devices areassociated with and controlled by the CPU 560 by optically isolatinginterfaces 562 and 564. In particular, the other CNC 126b interchanges aset of "handshaking" signals via a CNC link 558 and the interface 562with the CPU 560, whereby each of the CNC's 126a and 126b may bid forand obtain control of the beam switching mirror 172 in a time sharedfashion. As explained in the co-pending application entitled "PLURALCOMPUTER CONTROL FOR SHARED LASER MACHINING" (Ser. No. 414,204) each ofthe two CNC's 126a and 126b may bid for and subsequently control thebeam switching mirror 172 to direct the laser beam 178 into its weldingchamber 108. After use, the CNC 126 generates a release laser signal,whereby the other CNC 126 may request and subsequently lock the laserfor its own use.

The laser system 102 may in one illustrative embodiment of thisinvention take the form of that laser system as manufactured by Raytheonunder their model number SS500 and comprises the laser power supply 120as shown in FIG. 4, as well as a laser control system 592 that iscoupled by the interface 562 to the CPU 560. As shown in FIG. 22B, thelaser control system 592 is coupled to a laser welding display panel 132which, as shown in FIG. 4, is mounted upon the laser power supply 120and is shown in detail in FIG. 23A. The laser welding display panel 132includes an array of lamps and pushbuttons that control and display thecondition of the laser system 102 and its control system 592. Before thelaser rod 170 may be fired to emit its beam 177 of laser radiation, thelaser triggers must be turned on or enabled. An illuminated pushbutton600 is actuated to apply high voltage from the laser power supply 120 tothe pulse forming network, provided that the laser power supply is inits standby mode. When the laser power supply provides a high voltage,the LASER HV ON pushbutton 600 is illuminated. A SHUTTER OPEN lamp 602is illuminated when the dump shutter 190 is disposed in its openposition and the BRH safety shutter 212 is disposed in its openposition, whereby the laser beam 177 is directed into one of the weldingchambers 108 and the TV camera 206 is permitted to view the image of thefuel rod grid 16. A LASER FIRING lamp 604 is illuminated when the laserrod 170 is lasing, i.e. when its excitation lamps 186 are triggered, theinner-cavity shutter 188 opens and its CNC 126 has gained control of thelaser system 102. A BEAM SW IN position lamp 608 is illuminated when thebeam switching mirror 172 is disposed in position to direct the laserbeam into the right welding chamber 108a, whereas a BEAM SW OUT positionlamp 612 is illuminated when the beam switching mirror 172 is in itsalternate position, whereby the laser beam 177 is directed into theother, left welding chamber 108b. A GAS ON lamp 610 is illuminated whena particular argon gas flow rate has been selected by its CNC 126. AHOME MIRROR pushbutton 614 is pressed to direct the beam switchingmirror 172 to its home or reference position. A TRIGGERS ON lamp 616 ispressed to enable the laser lamp trigger circuits, provided that thelaser high voltage has been turned on. A LASER HV OFF pushbutton 618 ispressed to remove the high voltage output from the laser power supply120. The meters 498 and 492 are digital meters continuously displayingthe oxygen content and the water content of the welding chamber 108.

As seen in FIGS. 22A and 22B, the CPU 560 provides control signals viathe optically isolated interface 562 to actuate the laser control system592. In particular, interface outputs are applied to the laser controlsystem 592 to turn on or off the high voltage output of the power supply120, to enable the laser lamp triggers, to dispose the dump shutter 190and the BRH safety shutter 212 to their open positions, to initiate thewelding process, to select a particular mode of laser welding dependenton one of the codes M51 through M54, to set the pulse frequency (REPRATE) as derived from the T code, to set the power level as derived fromthe S code, to set the pulse width, and to position the beam switchingmirror 172. Signals are developed by the laser control system 592indicative of the completion of a weld as well as the laser status to beapplied via the optically isolated interface 562 to the CPU 560. Upongeneration of emergency stop signals, the operations of the laserwelding system 102 and in particular of the laser control system 592 maybe stopped in an emergency.

Further, signals are developed by the CPU 560 and are transmitted by theoptically isolated interface 562 to control the door opening mechanism234 as shown in FIG. 7 to either open or close the doors 114 of thecabinet 104. Signals are applied to lock or unlock the welding chamber108 and, in particular, are applied to each of the front and backlocator assemblies 284 and 286 as shown in FIG. 9. The output signals asderived from the sets 572, 574, and 576 of limit switches are applied tothe interface 562. Signals are also applied to a laser water coolingsystem 620. The laser flash or excitation lamps 186 and the cavity asdefined by the mirrors 182 and 184 are cooled by the closed-loop watercooling system which provides clean, pure, temperature-regulated waterat the required pressure and flow rate. Though not shown, it is wellunderstood in the art that the laser water cooling system includes apump, a water-to-water heat exchanger, a reservoir, a deionizer, afilter, and a temperature regulator. Heat from the laser rod 170 and thebeam absorber 194 is discharged to the water and removed from thesystem. In addition, a control signal is applied to the lamp 428 of thelaser lens assembly 204, to illuminate the fuel rod grid 16, whereby theX-Y positioning system 288 may be adjusted along either the X or Y axisto align the starting point of the fuel rod grid 16 with respect to thelaser beam 178.

Inputs are provided from the oxygen probe 496 and the moisture sensor410 that are disposed with respect to the welding chamber 108 to provideanalog signals indicative in parts per million of the oxygen and waterwithin the welding chamber atmosphere. In similar fashion, thethermopile 218 as disposed with the shield tube 216 provides an analogsignal indicative of the power of the laser beam 178 directed therein.The outputs of each of the probe 496, the sensor 410, and the thermopile218 are applied to corresponding digital volt meters 578, 580, and 582,which convert the input analog signals to corresponding digital signalsto be applied via the optically isolated interface 564 to the CPU 560.The interface 564 provides appropriate meter select signals to each ofthe digital volt meters 578, 580, and 582 to selectively apply only onedigital signal at a time via the interface 564 to the CPU 560. Dependingupon the operation of the laser welding system 100, the CPU 560 appliessignals via the optically isolated interface 564 to each of the massflow controllers 488, 484, and 486 to control the rate of argon flowrespectively to the laser lens assembly 204, the rotatable fixture 242and the welding chamber 108. In similar fashion, signals are applied tothe B axis motor 388, whereby the positioning wheel 358 and therotatable fixture 242 may be rotated. As explained above, the angularposition of the positioning wheel 358 is sensed by the plurality ofproximity switches 402a-d to provide a binary signal that is applied bythe interface 564 to the CPU 560.

Referring now to FIG. 23B, there is shown the machine function panel(MFP) 130 as mounted on the computer housing 129 as shown in FIG. 4 forproviding, as suggested in FIG. 22A, inputs via the optically isolatedinterface 564 to the CPU 560. The various control functions asimplemented by the pushbuttons and selector switches of the machinefunction panel 130 will now be described. The EMERGENCY STOP pushbutton680 is actuated by the operator in an emergency situation to turn offthe CNC 126. When pressed, all digital outputs as derived from the CPU560 are deactived, and all auxiliary systems such as the argon supplysystem 473, the laser system 102, and the X and Y drives 294 and 296,and the Z drive 470 are stopped. A CONTROL ON pushbutton 668 is actuatedto turn the CNC 126 to its ON state, whereby power is applied to thevarious logic elements and various data registers are cleared. When thepushbutton 668 is pressed and held, the lamps back-lighting various ofthe push buttons of the machine function panel 130 are energized toprovide a suitable test thereof. A CLEAR pushbutton 656 is actuated toclear the CNC 126 and in particular to clear all active commands asstored in the stored program active buffer of the CPU 560, and selectedoutputs thereof are reset. The M and G codes as set in the course of theparts program are reset to initial conditions. In the course of theexecution of the various programs, the pushbutton 656 is illuminated torequest a clear function from the operator. A MESSAGE pushbutton 638 isperiodically illuminated or flashed to indicate that there is adiagnostic message to be displayed on the CRT 133. When pressed by theoperator, all active diagnostic messages are cleared from the displayand the lamp illuminating the push button 638 deactuated. A TEST I lamp636 is illuminated to indicate that the welding chamber 108 is disposedin its second, out or calibrating position and that the cooling waterdirected to the thermopile 218 has been turned on. A SERVO ON pushbutton666 is operator actuated to apply AC power to the X and Y axes drivemotors 294 and 296, and the Z axis drive motor 470, and is illuminatedwhen these drives are active. A PIN OUT pushbutton 634 is pressed andheld by the operator to actuate the front and back locator assemblies284 and 286, whereby their locating pins 316 and 319 are removed toun-pin the slide table 262 for subsequent movement. The CNC 126 must bein its manual mode to permit this function. The PIN OUT pushbutton 634is illuminated when the locating pins 316 and 319 are fully retracted.

A PIN IN pushbutton 652 when pressed and held by the operator actuatesthe front and back locator assemblies 284 and 286 to dispose theirlocating pins 316 and 319 into the positioning openings within the slidetable 262. Similarly, the CNC 126 must be in its MANUAL mode to permitthis function. When the locating pins 316 and 319 are fully insertedinto their positioning openings, the PIN IN pushbutton 652 isilluminated. A DOOR OPEN pushbutton 632 is pressed and held by theoperator to actuate the door opening mechanism 234. The CNC 126 must bein its MANUAL mode to permit this function to be carried out; when thedoor 114 has been disposed to its fully open position, the DOOR OPENpushbutton 632 is illuminated. A DOOR CLOSE pushbutton 650 is pressedand held to activate the door opening mechanism 234 to close the chamberdoor 114. The CNC 126 must be in its MANUAL mode to permit thisfunction. When the cabinet door 114 is disposed to its completely closedposition, the DOOR CLOSE pushbutton 650 is illuminated. A CHAMBER OUTpushbutton 630 is pressed and held by the operator to actuate the slidedrive motor 266, whereby the slide table 262 and its welding chamber 108are driven to its second, out position. In order to drive the slidetable 262, the CNC 126 must be in its MANUAL mode, the laser lensassembly 204 must be fully retracted as sensed by the limit switch 576b,and the Y positioning table 292 must be disposed in its center positionas detected by the limit switch 574d. When the slide table 262 has beendisposed in its second, out position, the CHAMBER OUT pushbutton 630 isilluminated. In similar fashion, a DOOR CLOSE pushbutton 650 is pressedand held to actuate the slide drive motor 266 in reverse direction toreturn the slide table 262 to its first, welding position. In order todrive the slide table 262 in, the CNC 126 must be in its MANUAL mode,the laser lens assembly 204 fully retracted, the door 114 open, thefront and back locator assemblies 284 and 286 actuated to withdraw theirlocating pins, and the Y positioning table 292 centered. When the slidetable 262 has been disposed in its first, welding position, the CHAMBERIN pushbutton 648 is illuminated.

A FEEDHOLD pushbutton 660 is initially pressed to set the FEEDHOLDfunction, whereby each of the X, Y, and Z drive motors 294, 296, and 470is deactuated; as a result, the movement of the welding chamber 108along its X or Y axes, except for the movement of the rotatable fixture242 about its B axis, and the movement of the laser lens assembly 204along its Z axis are inhibited. Upon a second pressing of the FEEDHOLDpushbutton 660, the FEEDHOLD function is released, thereby permittingthe movement of the welding chamber 108 along its X and Y axes and themovement of the laser lens assembly 204 along its Z axis. A CYCLE STARTpushbutton 658 is actuated by the operator to initiate execution of thepart program data when the CNC 126 is in its AUTO, SINGLE CYCLE orMANUAL data input MDI modes. The CYCLE START pushbutton 658 will beilluminated when the CNC 126 is executing part program data. A MANUALpushbutton 678 is pressed to place the CNC 126 in its MANUAL mode ofoperation; when in its MANUAL mode, the MANUAL pushbutton 678 isilluminated. An MDI SINGLE pushbutton 676 is pushed by the operator todispose the CNC 126 to its manual data input MDI SINGLE mode ofoperation; when in the MDI SINGLE mode, the pushbutton 676 isilluminated. The MDI SINGLE mode is a diagnostic tool and when thisfunction is entered, the operator is permitted to enter steps of a partprogram via the keyboard 131 into a designated area or buffer of the CPUmemory. Upon depression of the CYCLE START pushbutton 658, the enteredprogram is read out and executed one step at a time. An MDI CONTINUOUSpushbutton 674 is pushed to dispose the CNC 126 in its MDI CONTINUOUSmode of operation. The MDI CONTINUOUS mode is similar to the MDI SINGLEmode, except that upon depression of the START CYCLE pushbutton 658, theentire operator entered program is read out as if in its automatic mode.A SINGLE CYCLE pushbutton 672 is pressed by the operator to dispose CNC126 into it SINGLE CYCLE mode and when in that mode, the pushbutton 672is illuminated. An AUTO pushbutton 670 is depressed to place the CNC 126in its AUTOMATIC mode of operation and when in that mode, the AUTOpushbutton 670 is illuminated.

A % FEED selector switch 682 has twelve positions to provide a manualoverride of the drive feed rate at which the X and Y drive motors 294and 296 drive respectively the X and Y positioning tables 290 and 292.As indicated, the feed rate is variable in 10% increments from 10% thru120% dependent upon the position of the percent FEED switch 682. A JOGMODE selector switch 684 has seven positions for selecting one of thefollowing axis JOG MODES: FAST (HIGH), SLOW (LOW), 1.000, 0.1000,0.0100, 0.0010, and 0.0001. The FAST and SLOW modes are "slew" type jogswhere a substantially continuous movement is imparted to the X and Ypositioning tables 290 and 292, whereas in the remaining modes,incremental movements of the designated length are imparted to the X andY positioning tables 290 and 292. An X IN pushbutton 622 is pressed bythe operator to cause an X-axis jog motion in the minus direction orinto the positioning module 106, i.e. in an upward direction as shown inFIG. 9. An X OUT pushbutton 640 is pressed to cause an X-axis jog motionin the plus direction or out of the positioning module 106, i.e. in adownward motion as shown in FIG. 9. A Y LEFT pushbutton 624 is pressedby the operator to cause a Y-axis jog motion in the plus or leftdirection, i.e. the welding chamber 108 is moved to the left as seen inFIG. 9. A Y RIGHT pushbutton 642 is pressed by the operator to cause aY-axis jog motion in the minus or RIGHT direction, i.e., the weldingchamber 108 is moved to the right as seen in FIG. 9.

A Z UP pushbutton 626 is depressed by the operator to cause a Z-axis jogmotion in the minus direction; i.e. the Z axis drive motor 470 isenergized to drive the Z axis table 458 in a minus or UP direction asshown in FIG. 7. A Z DOWN pushbutton 644 is pressed by the operator tocause a Z-axis jog motion in the plus direction, whereby the Z-axistable 458 and the laser lens assembly 204 carried thereby are disposedin a plus or DOWN direction as seen in FIG. 7. A B CW pushbutton 628 ispressed to cause a B axis motion in the plus or fixture clockwise motionif the CNC 126 is disposed in its MANUAL mode. In particular, uponpressing of the pushbutton 628, the B axis drive motor 388 is actuatedto rotate the positioning wheel 358 in its clockwise direction as shownin FIG. 7. A B CCW pushbutton 646 is depressed by the operator to causea B axis motion in the minus or counterclockwise direction if the CNC126 is disposed in its MANUAL mode. In particular, the B-axis drivemotor 388 is actuated to drive the positioning wheel 358 in thecounterclockwise direction as seen in FIG. 7.

The process of welding the inner grid straps 20 together and in turn tothe outer grid straps 22 and the resultant grid 16 to the guide sleeves36 has been described above with respect to FIGS. 3A to 3K; in thesefigures, there is illustrated the series of movements of the fuel rodgrid 16 in each of its X, Y, and Z axes to appropriately position thefuel rod grid 16 with respect to the laser beam 178, whereby each of theintersect welds 32, the corner seam welds 30, the slot and tab welds 34,and the notch seam welds 40 may be effected. The inner and outer gridstraps 20 and 22 are assembled to form the fuel rod grid 16 as explainedin the copending applications entitled "STRAP AND VANE POSITIONINGFIXTURE FOR FUEL ROD GRID AND METHOD" (Ser. No. 414,197) and "GRIDASSEMBLY FIXTURE, RETENTION STRAP AND METHOD" (Ser. No. 414,198). Next,the fuel rod grid 16 is disposed upon the welding fixture 542 shown inFIG. 15 and described in the copending application entitled "WELDINGPLATES FOR A FUEL ROD GRID" (Ser. No. 414,265); the welding fixture 542in turn is releasably affixed by the locating pins 524 to the rotatablefixture 242 rotatably disposed within the welding chamber 108. Asexplained above, the fuel rod grid 16 may be rotated about its B axis todispose the fuel rod grid 16 in position to receive the laser beam 178to effect the notch seam welds 40. The X-Y positioning system 288 isselectively actuated to move the X and Y positioning tables 290 and 292in a sequence of incremental steps along their X and Y axes to positionthe fuel rod grid 16 with respect to the laser beam 178, whereby theintersect welds 32 are effected, and after rotation upon the rotatablefixture 242, the slot and tab welds 34 and the corner seam welds 30 areeffected.

The machine control for this process is provided by the CNC 126 and inparticular by the CPU 560 which includes a memory for storing the partprogram 700, which will now be described with respect to FIGS. 24A and24B. The part program 700 is entered when in step 702, the operatorplaces the CNC 126 in its automatic mode by pressing the AUTO pushbutton670 on the machine function panel 130. Next, the operator enters acommand on the alpha-numeric keyboard 131 panel to call the part programfor execution. Next, the operator presses the CYCLE START pushbutton658. Next, in step 708. A programmed M81 code calls a LOAD/UNLOADCHAMBER application subroutine to effect the actuation of the slidedrive motor 266 to drive the slide table 262 from its first welding toits second, out position, whereby an operator may load an assembled,though not yet welded fuel rod grid 16 and its welding fixture 542 ontothe rotatable fixture 242. The fuel rod grid 16 and its welding fixture542 are locked by the locating pins 524 in a predetermined position onthe rotatable fixture 242 with respect to the laser beam 178. TheLOAD/UNLOAD CHAMBER subroutine is explained in greater detail withrespect to FIG. 25B. In step 710, the operator loads the fuel rod grid16 and its welding fixture 542 onto the rotatable fixture 242, with theassistance of the load/unload manipulator as described in the copendingapplication entitled "WORK PIECE GRIPPING AND MANIPULATING APPARATUS FORLASER WELDING SYSTEMS AND THE LIKE" (Ser. No. 414,260). At the end ofstep 708, the execution of the part program is suspended until in step712, the operator presses the CYCLE START pushbutton 658 to recommencethe execution of the part program. Next, step 714 calls the LOAD/UNLOADapplication subroutine to reload the chamber 108 into its first orwelding position beneath the laser beam 178. Once repositioned, an Mcode is used to call the CHAMBER ENVIRONMENT CHECK applicationsubroutine before the welding chamber 108 is purged of impurities suchas oxygen and water by introducing argon at a relatively high rate viathe manifold tubes 336 and the diffuser plate 330, whereby the heavierargon displaces the air driving it out through the spacing between thechamber's upper flange 331 and the sealing plate 156. The particularargon flow rate is set by an M code, whereby the mass flow controller484 is set to provide a high rate of flow to the welding chamber 108. Insimilar fashion, the mass flow controllers 486 and 488 associated withthe rotatable fixture 242 and the laser lens assembly 204 are set to ahigher flow rate to hasten the purging of the welding chamber 108. Theparticular M code calls the SELECT GAS FLOW RATE application routine aswill be further described with respect to FIG. 25I. Next, step 716 ofthe part program sets the M91 codes to effect rotation of the rotatablefixture 242 and in particular to actuate the B axis rotation drive 238to effect rotation of the fixture 242. In particular, the M91 code asexecuted by step 716 bids the ROTATE FIXTURE application subroutine aswill be described in greater detail in FIG. 25R. Step 718 serves toinitiate or bid the CHAMBER ENVIRONMENT CHECK application subroutine tomonitor the environment within the welding chamber 108 as to its oxygenand water content and to prevent the further execution of the partprogram until the levels of oxygen and water are below predeterminedlevels. The CHAMBER ENVIRONMENT CHECK application subroutine will befurther described with respect to FIG. 25F.

After step 718 has determined that the environment within the weldingchamber 108 is sufficiently pure, step 720 responds to X and Y codes tocontrollably drive the X and Y positioning tables 290 and 292, wherebythe initial weld to be made is positioned along the Z axis coincidingwith the laser beam 178. The initial welding position is identified by aset of X and Y codes which are interpreted to provide appropriatecontrol signals to the X and Y drive motors 294 and 296. In similarfashion, a Z code is interpreted and control signals are applied to theZ axis drive motor 470, whereby the laser lens assembly 204 ispositioned to focus the laser beam 178 onto the initial weld of the fuelrod grid 16. After completion of these steps, step 720 brings the partprogram to a stop. In step 722, the operator may manually control byappropriate actuation of the X IN pushbutton 622, the X OUT pushbutton640, the Y LEFT pushbutton 624 and the Y RIGHT pushbutton 642, theposition of the X and Y positioning tables 290 and 292, whereby theinitial weld of the fuel rod grid 16 is precisely aligned with respectto the laser beam 178. To this end, the BRH safety shutter 212 isopened, permitting the operator to view the grid image as displayed uponthe CRT 133 and obtained from the alignment TV camera 206. The lens ofthe camera 206 has an electronic rectical by which the operator mayalign the initial weld precisely with respect to the laser beam 178. Insimilar fashion, the operator manipulates the Z UP pushbutton 626 andthe Z DOWN pushbutton 644 to control the movement of the laser lensassembly 204 to precisely place the laser lens 202, whereby the laserbeam 178 is focused onto the fuel rod grid 16.

In order to reinitiate the execution of the parts program, the operatorin step 724 presses the CYCLE START pushbutton 658. Next, in step 726,the part program calculates the differences between the X and Ycoordinates of the initial weld position and of the aligned position,i.e. the new grid position after being aligned in step 722, thedifferences being known as the X and Y offsets. Similarly, thedifference between the initial home position along the Z-axis and thefocused position of the laser lens assembly 204 provides a Z offset. TheX, Y and Z offsets are stored in a designated area in memory and areused by the CNC 126 to calculate the precise position of each weldtaking into account the adjusted or offset position of the fuel rod grid16.

Next, step 728 sets the various parameters of the laser system 102 andin particular programs the S, T, and M codes that determine the powerlevel, the pulse frequency, the pulse width, and the type of weld, i.e.which of the intersect welds 32, the corner seam welds 30, the slot andtab welds 34, and the notch seam welds 40, to be performed. Inparticular, the power level of the laser system 102 is determined by anS code which is serviced by a SERVICE S CODE application subroutine aswill be explained in detail with respect to FIG. 25J. In similarfashion, the pulse frequency is set by a T CODE which is serviced by theSERVICE T CODE application subroutine as will be explained in detaillater with respect to FIG. 25K. The pulse width is set by one of the MCODES M55-M60 corresponding to widths of 1 to 6 ms, which bid theexecution of the SET LASER PULSE WIDTH application subroutine as shownin FIG. 25L. In similar fashion, there are four types of weldscorresponding to the M codes M51 through M54, which bid the execution ofthe SET LASER MODE application subroutine as will be explained ingreater detail in FIG. 25G. Next, step 730 sets by use of one of the MCODES M61 through M64 the particular argon flow rate that is requiredfor a welding operation and in particular bids the SELECT GAS FLOW RATEapplication subroutine, as will be explained in detail later withrespect to FIG. 25I. Next, in step 732, the set one of the M codes M51through M54 bids the PERFORM LASER WELD application subroutine, as willbe explained in greater detail with respect to FIG. 25N. Generally, thePERFORM LASER WELD application subroutine first requests or bids for theuse of the laser via the GET LASER application subroutine as shown inFIG. 25Q, whereby the other CNC 126b is checked by examining the REQUESTLASER and LOCK LASER outputs of the other CNC 126b and if present, theCNC 126a waits until the appearance of a RELEASE LASER output from theother CNC 126b, at which time the CNC 126a requests and thereafter locksthe laser for its use. Upon obtaining the use of the laser system 102,the CNC 126a disposes the beam switching mirrors 172 to direct the laserbeam 178 to its welding chamber 108. Thereafter, the positions of the Xand Y positioning tables 290 and 292 are checked to see if they havecome to rest in their proper position and a positioning time out periodis permitted to expire before firing the laser rod 170. Then, step 732waits for a LASING COMPLETE signal indicating that the welding step hasbeen completed before releasing the beam switching mirror 172 andcommanding the X-Y positioning system 288 to move the fuel rod grid 16to its next position in preparation for performing the next in a seriesof welds. Next, step 736 decides whether the particular type of weld asset by one of the M codes M51 through M54 has been completed and if not,the part program returns to step 732 to perform the next weld and thenin step 734, to move the fuel rod grid 16 to its next weld position.Thereafter, step 735 determines whether the M code M88 has beenprogrammed to bid for the WAIT FOR OTHER CNC application subroutine,whereby a signal is transmitted to the other CNC 126b to indicate that aseries of welds has been completed and then to wait for a response fromthe other CNC 126b; during this interval, the part program execution issuspended.

After a particular type of weld has been completed, the part programmoves to step 738 where the part program stops and examines which of theM codes M51 through M54 has been programmed to determine the next typeof weld. Thereafter, in step 740, a decision is made as to whether allof the types of welds necessary to complete the welding of at least oneside of the fuel rod grid 16 has been made and if not, the part programreturns to step 716, whereby the sequence of steps 716 through 738 isrepeated. The first sequence of welding steps as illustrated in FIGS. 3Ato 3D is carried out on the vane side of the nuclear fuel rod grid 16before it is then necessary to remove the fuel rod grid 16 from itswelding chamber 108 to be rotated and returned to the welding chamber108. In step 742, the laser system 102 is turned off by sending a signalto dispose the dump shutter 190 to a position as shown in full line ofFIG. 6 to direct the laser beam 177 into the laser absorber 194.

Thereafter, step 744 sets the M code M82 to bid for the LOAD/UNLOAD CARTapplication subroutine, whereby the slide drive motor 266 is actuated todirect the slide table 262 to its second, out position, whereby the fuelrod grid 16 may be removed from the welding chamber 108. At this point,the operator brings the manual manipulator to remove the fuel rod grid16 and its welding fixture 542 from the welding chamber 108 to performthose manual operations in preparation for the next sequence of weldingsteps. For example, after the intersect welds 32 on the vane side of thefuel rod grid 16 are completed as in the steps shown in FIGS. 3A to 3D,the fuel rod grid 16 is removed and rotated so that the intersect welds32 as appearing on the opposite or guide sleeve side of the fuel rodgrid 16 may be completed as seen in the steps of FIGS. 3E to 3H. Afterthe intersect welds on both sides of the fuel rod grid 16 have beencompleted, the grid 16 is removed and the guide sleeves 36 are insertedtherein, before effecting the notch seam welds 40 as shown in the steps3I to 3L.

Referring now to FIG. 25A, there is shown a CALIBRATION part programwhereby the operator may manually enter via the alpha-numeric keyboard131, a command to call the CALIBRATION part program. First in step 750,the operator presses the AUTO pushbutton 670. Thereafter in step 752,the operator types on the keyboard 131, a command calling theCALIBRATION part program and then in step 754, presses the CYCLE STARTpushbutton 658. Next, step 756 bids for the LOAD/UNLOAD CART applicationsubroutine, whereby the slide table 262 and therefore the weldingchamber 108 are disposed to their second, out position, whereby as shownin solid line in FIG. 6, the laser beam 178 is directed into the shieldtube 216 and onto the thermopile 218. Next, in step 758, the CNC 126controls the Z drive motor 470 to cause the laser lens assembly 204 tofocus the laser beam into the termopile 218. The laser system 102 is setup in step 728', which performs the same operations as step 728 as shownin FIG. 24B, i.e. to set the power level according to its S code, thepulse frequency according to its T code, the pulse width according toits M code, and the weld type according to its M code, before in step759 calling the CALIBRATION application subroutine. In particular, step759 sets the M code M98, whereby the LASER CALIBRATION applicationsubroutine is bid. In this way, the operator at selected times mayeffect a calibration of the laser system 102 noting that the efficiencyof the laser rod 170 and its excitation lamps 186 attenuate with intenseuse of this laser welding system 100, i.e. the laser welding system 100continuously excites the laser rod 170 to thereby ensure a high workduty ratio and as a result, a high production rate of the fuel rod grids16. It is contemplated that the laser excitation lamps 186 may need tobe replaced as often as every two days. In the course of continuous use,the operator will wish to check the actual power output of the laser rod170 and to measure by the thermopile 218 its energy level as impingingon the work piece, whereby an adjustment of the output of the laser highvoltage supply may be effected to maintain the intensity of the laserbeam 178 at a level whereby uniform welds are achieved, regardless ofthe work life of the laser rod 170 or its excitation lamps 186.

The laser CALIBRATION application subroutine as shown in FIG. 25M isentered by the setting of the M code M98 in step 759 and generallycalculates the reservoir voltage RESVOLT as applied to the pulse formingnetwork (PFN), whereby its output voltage as applied to the excitationlamps 186 is adjusted for the programmed power level according to theselected S code, pulse width, and pulse repetition rate (T code). Thelaser CALIBRATION application subroutine then fires the laser rod 170,takes a reading of the output of the thermopile 218, and compares thethermopile output with the programmed power level; the differencebetween the calculated and measured output of the thermopile 218 is usedto adjust the reservoir voltage output. This reiteration processcontinues until the measured and programmed laser power levels arewithin a predetermined difference, e.g. 2 watts. When divergence isreached, the new value of the reservoir voltage is stored at a specifiedlocation within the memory of the CPU 560. Initially after entry in step759, step 1022 solves equations 1 and 2 for a value of the reservoirvoltage (RESVOLT) based upon the particular characteristics of the pulseforming network of a particular illustrative laser system, namely themodel number SS500 as manufactured by Raytheon. Calculations have beencarried out for the particular pulse forming network of the notedRaytheon laser system to provide a curve as shown in FIG. 25S that wascalculated to provide parameters characterizing the pulse formingnetwork factor as a function of the repetition rate or REP RATE. Theparameter M in equation 1 is defined as the slope of the empiricallyderived curve of FIG. 25S, and the offset of the curve at zero REP RATEis taken for this pulse forming network as a value of 57; the slope Mhas an illustrative value of 0.33. The value of the pulse formingnetwork factor PENFACTR is calculated in equation 1 and is substitutedinto equation 2 along with suitable values of the desired power outputof the laser beam 178 in accordance with the programmed S code, the REPRATE as determined by the T code, and the pulse width as determined bythe selected M code to provide in accordance with the terms of equation2 a calculated value of the reservoir voltage RESVOLT. Next, step 1024scales the calculated value of RESVOLT for the digital to analog D/Acircuit of the particular Raytheon laser system 102. Next, thecalculated value of RESVOLT is checked by a safe power level SAFPWRapplication subroutine that is called and executed as will be explainedin detail as with respect to FIG. 250. If the calculated RESVOLT issafe, step 1028 energizes the pulse forming network and after a suitabledelay as timed in step 1030, step 1032 actuates the beam switchingmirror 172 to direct the laser beam 178 to the welding chamber 108 beingcontrolled. Next, step 1034 determines whether the excitation lamps 186are energized and if not, an alarm message is displayed by step 1036 onthe CRT 133. If the excitation lamps 186 have been turned on, thetrigger circuit associated with the excitation lamps 186 is enabledbefore step 1040 opens the dump shutter 190. Next, step 1042 opens theinnercavity shutter 188 to permit the laser rod 170 to emit the laserbeam into the thermopile 218. Step 1044 times a suitable period beforeclosing the innercavity shutter 188, accesses the output of thermopile218 and converts the analog thermopile output to a corresponding digitalmanifestation. Step 1046 compares the measured laser power level withthe programmed value thereof in accordance with the S code and if thedifference is within 2 watts plus or minus, the corresponding value ofthe reservoir voltage RESVOLT is stored in a table of the memory of theCPU 560. If divergence is not met, step 1050 determines whether this isthe 6th loop of the steps 1022 to 1046, and if so, step 1052 displaysupon the CRT 133 an alarm message that the laser CALIBRATION applicationsubroutine is unable to reach divergence. If less than the 6th loop,step 1054 calculates an offset or modified value S3 of the S code inaccordance with the equation shown in step 1054 of FIG. 25M, where S1 isthe initially programmed S code, MEAS POWER is the power of the laserbeam 178 measured in step 1046 by the thermopile 218, and S2 is thepreviously calculated offset. The modified code S3 is returned and usedin step 1022 to calculate a new value of RESVOLT in accordance with thesecond equation. Thereafter, steps 1024 through steps 1054 are repeateduntil divergence is obtained or six loops have been completed. As shownin FIG. 25M, when divergence is obtained as determined in step 1046, theoutput voltage from the power supply as applied to the pulse formingnetwork of the laser system 102 is stored as an offset value in a tablein memory, whereby a compensation of the laser output beam iseffectively made, and the energy input for each weld is heldsubstantially constant over a long period of time thus assuring welduniformity.

The LOAD/UNLOAD CART application subroutine is shown in FIG. 25B foractuating the slide drive motor 266 to dispose the slide table 262 andits welding chamber 108 between its first, (welding) position in, andits second, out position, while insuring that the door 114 is open, thelaser lens assembly 204 is retracted, and the locating pins 316 and 319are removed, permitting the slide table 262 to move. Initially in step760, the M code as set in step 708 of the part program as shown in FIG.24A is executed during the Bidflag Execute Cycle of the operating systemprogram. In particular, step 708 sets an M code M82 to unload the slidetable 262 and its welding chamber 108, whereas in step 710, an M codeM81 is set whereby the slide table 262 is returned to its first, weldingposition. Next, step 762 accesses the safety zone 134 in front of thewelding chamber 108 to be moved and if free, step 764 actuates the Zdrive motor 470 to move the laser lens assembly 204 to its homeposition. Next, step 766 actuates the X and Y drive motors 294 and 296to dispose the X and Y positioning tables 290 and 292 to their centerposition and to their home or extended position, respectively. Next,step 768 sets the FEED HOLD to bring the X and Y positioning tables 290and 292 to a halt, and the door opening mechanism 234 is actuated todispose the door 114 to its open position. Next, the front and backlocator assemblies 284 and 286 are actuated to raise their locating pins316 and 319, thereby freeing the slide table 262. Thereafter, step 772actuates the slide drive motor 266 to direct the slide table 262outwardly when an M code M82 has been set or inwardly when an M code M81has been set. Then, step 774 actuates the front and back locatorassemblies 284 and 286 to dispose their locator pins 316 and 319 into alocking position with respect to the slide table 262. Next, the cabinetdoor 114 is closed in response to the M code M81, and in step 780, theFEED HOLD is released. In step 782, a decision is made as to whether theM code M81 has been set indicating that the welding chamber 108 is to beloaded and if so, the CHAMBER ENVIRONMENT CHECK application subroutineas shown in FIG. 25F is bid to ensure that the atmosphere within thewelding chamber 108 is of sufficient purity to permit welding.Thereafter, step 784 clears the routines Bidflag and sequence pointerbefore exiting.

Referring now to FIG. 25C, there is shown the SYNC applicationsubroutine that is called in step 790 by the 100 Hz clock of the CPU560. Step 792 determines whether the beam switching mirror 172 is in theposition that would direct the laser beam 178 to the welding chamberassociated with the other CNC 126b and has not been commanded to changeits position. If no, step 793 deselects the laser's operating voltage bythe parent CNC 126a. It is understood that each of the parent CNC 126aand the other CNC 126b time shares the control of the laser system 102,wherein only one CNC 126 has control at any one instant of time.Further, one of the CNC's 126a and 126b is designated as the prime CNCand sets the pulse width and REP RATE, which parameters the other CNCadopts. However, each CNC 126a and 126b performs its own calibrationprocess that is a function of the optical path that the laser beams 178aand 178b travel into the corresponding welding chambers 108a and 108b.In this regard, it is understood that the degree of attenuation impartedby each optical system to its laser beam 178 differs to some degree andfurther the welding process occurring in each welding chamber 108 mayhave a different effect, i.e. the coating of its laser focusing lens202. After calibration, each CNC 126a and 126b will calculate its ownvalue of the reservoir voltage RESVOLT that is applied via its owndigital/analog converter to the laser control system 592, as shown inFIG. 22B. Thus, it is necessary when one of the CNC's 126a or 126 breleases control of the laser system 102, that it deactuates itsdigital/analog converter as would otherwise apply its operating orreservoir voltage to the laser control system 592; in other words, eachCNC 126 deselects its operating voltage as indicated in step 793. Next,step 794 determines whether both CNC's 126a and 126b have currentlylocked the laser, and, if yes, step 795 determines whether the parentCNC 126a has locked the laser. If no, step 796 causes the parent CNC126a to unlock the laser and to re-initialize the GET LASER applicationsubroutine, as shown in FIG. 25Q. Next, step 798 determines whether theparent CNC 126a has both locked and requested the laser and if not, theparent CNC 126 resets the laser release signal as applied in step 800via the CNC link 558 to the other CNC 126b as shown in FIG. 22, beforeproceeding directly to step 808. If yes, step 802 determines whether theother CNC 126b has applied a laser request signal via the CNC link 558to the parent CNC 126a and if yes, the routine moves to step 804 whichdecides whether the parent CNC 126a has locked the laser system 102 byapplying a laser lock signal to the CNC link 558; if not, the parent CNC126a updates in step 806 the CNC link 558, resetting its laser requestsignal before proceeding to step 808.

When the parent CNC 126a has locked the laser, i.e. has applied a laserlock signal via the CNC link 558 to the other CNC 126b, the routineinitiates a laser status check. Initially, step 808 determines whetherthe parent CNC 126a has control over the laser, i.e. is the busy flaggenerated, and if not, the subroutine exits. If yes, the applicationsubroutine checks in step 810 various laser parameters including whetherthe lamp voltage is on, whether the excitation lamps 186 are operatingat their less than maximum power limit, the temperature of the lampcoolant, the flow rate of the lamp coolant, the current and voltage asdrawn by the laser power supply, and whether the cabinet door is open.If all are OK, an exit is made from this subroutine. If any of the lampparameters are not within limits, step 812 determines whether the lasertrigger circuits are enabled and if yes, step 814 turns off the laserenabling circuits before setting a delay to disable the laser system102. If step 812 decides no, step 818 determines whether the innercavityshutter 188 is open and if yes, step 820 sets the alarm status beforestep 822 disposes the beam switching mirror 172 out of position and step824 displays a suitable alarm message "MIRROR OUT OF POSITION" on theCRT 133. If the innercavity shutter 188 is closed, step 826 immediatelydisplays the alarm message on the CRT 133.

The MAIN application subroutine is shown in FIG. 25D and operates withinthe high level operating system program or the MAIN TASK LOOP. The mainroutine continuously operates, except when interrupted by a flag orother interrupt as other would be set by the keyboard 131, the papertape reader 584, or the magnetic tape drive 586. As noted above, theSYNC routine will be called by an interrupt generated by the CPU's 100Hz sync clock signal. Initially, step 830 looks for flags set during theSYNC application subroutine. Step 832 determines whether the TriggerFlag is set and if so, a Trigger Timer is allowed to time out before theTrigger enable output is cleared in step 834, whereby the triggercircuit associated with the laser system 102 is cleared. Next, step 836checks a designated bit 15 of the Alarm Flag indicating that the beamswitching mirror 172 is out of position and if yes, step 838 displays onthe CRT 133 the message "MIRROR OUT OF POSITION". Step 840 determineswhether the MESSAGE pushbutton 638 on the machine function panel 130 asshown in FIG. 23b has been actuated and if so, the alarm and errormessage flags are reset to acknowledge all error conditions and ifpresent, the MESSAGE pushbutton 638 is illuminated. Finally, in step842, if a program stop as would occur in the parts program as shown inFIGS. 24A and 24B is set, the laser dump shutter output is cleared,whereby the dump shutter 190 is closed to direct the laser beam 177 intothe beam absorber 194.

Referring now to FIG. 25E, there is shown the CLEAR applicationsubroutine which is executed within the CLEAR Bidflag function of theoperating system program to clear all application timers, variables,outputs, and Bidflags contained in the Initialize Table of the memory ofthe CPU 560. This is effected as indicated in step 850 by addressing thememory's Initialize Table. In step 852, its length is determined wherebythe value stored therein may be cleared before exiting. It is understoodthat Bidflags are the means by which the M codes when executed by theoperating system bid or call the application subroutines as explained inFIGS. 25A to 25R.

The CHAMBER ENVIRONMENT CHECK application subroutine is shown in FIG.25F and is bid from step 718 of the part program shown in FIG. 24A, thebid being executed during the next Bidflag execute cycle of the maintask loop or operating system program. Basically, this applicationsubroutine reads the moisture sensor 410 and the oxygen probe 496disposed to take readings of oxygen and water of the atmosphere withinthe welding chamber 108 to determine whether the welding atmosphere issufficiently pure. The readings of oxygen and water are compared withspecified limits to determine the "GO-NO GO" chamber environmentcondition for laser welding. In step 862, the "GO-NO GO" flag iscleared. Thereafter, step 864 displays on the CRT 133 of the CNC 126,the message "Chamber Gas Check" indicating that the oxygen and watercontent of the chamber environment is being measured. Thereafter, step866 accesses the selected oxygen probe 496 and sets a time delay inwhich sample oxygen readings are taken before the CPU 560 takes itsreading. In similar fashion, step 868 selects the desired moisturesensor 410 and sets a delay during which samples thereof are made beforethe CPU 560 takes its reading. Next, step 870 converts the analogreadings of the oxygen probe 496 and the moisture sensor 410 to digitalvalues, and compares these values with the preselected limits. Step 872determines whether the oxygen and moisture values are less than the setlimits and if not, step 874 sets a delay before the next set of waterand oxygen readings are taken, and step 876 causes an alarm message "BadReading" to be displayed upon the CRT in Step 876. If a bad or abovelimit reading is made, steps 864 to 872 are repeated, contemplating thatas the air is purged from the welding chamber 108, the desired degree ofpurity will be achieved within a relatively short period of time, e.g.one minute. If a single good reading is made as determined by step 872,step 880 determines whether three such good readings have been made andif not, steps 864 to 872 are repeated. After three good readings havebeen made as determined by step 880, step 882 sets the GO conditionalflag before returning to the part program and in particular to step 720as seen FIG. 24A.

FIG. 25G shows the SET LASER MODE application subroutine that is bidfrom step 728 of the part program shown in FIG. 24B and is executedduring the Bidflag Execute Cycle of the main task loop or operatingsystem program. The SET LASER MODE application subroutine determineswhich of the four laser modes corresponding to the four different typesof welds made upon the fuel rod grid 16 is to be selected. As shown inFIG. 2A, there are four different types of welds, namely the intersectwelds 32, the corner seam welds 30, the slot and tab welds 34, and thenotch seam welds 40. As explained above, the laser system 102illustratively taking the form of a model number SS500 as manufacturedby Raytheon, comprises four distinct modules or hard wired circuits forcontrolling each of the four types of welds, as to the number of pulsesand/or the time interval during which the pulse laser beam 178 isdirected onto the fuel rod grid 16. Illustratively, each of thesemodules has thumb wheel switches to permit the setting of the number ofpulses or time period in which the laser beam 178 is directed onto thework piece. Each module and its weld type is assigned one of the codesM51 through M54. In step 902, one of these modules is addressedaccording to its code. In step 904, the laser mode output as appears onthe SELECT LASER MODE terminal of the optically isolated interface 562is cleared. Finally, step 906 applies the selected mode via the SELECTLASER MODE OUTPUT to the laser control system 592 to select the desiredmodule for the particular type of weld. As will be explained later, thepulse width and the frequency or REP RATE are selected in accordancewith programmed M and T codes.

Referring now to FIG. 25H, the SET LASER PULSE WIDTH applicationsubroutine is shown. Initially in step 910, this application subroutineis entered by setting one of the M codes M55 through M59 dependent uponthe selected one of possible five laser pulse widths in step 728 of thepart program shown in FIG. 24B and is executed subsequently during thenext Bidflag Execute Cycle Step 912 which interprets and accesses theselected M code in the data pool of the memory of the CPU 560. Step 914checks the safe power level of the laser beam 178 as calculated with theselected pulse width by biding the SAFEPWR application subroutine aswill be described in detail in FIG. 25O. Step 916 determines whether thecalculated power level is safe, i.e. less than maximum limits, and ifnot, step 918 sets an alarm whereby an immediate stop to the partprogram is effected. If safe, step 920 resets the SELECT PULSE WIDTHOUTPUT of the interface 562, and step 922 sets the SELECTED PULSE WIDTHOUTPUT, whereby the laser control system 592 sets the desired pulsewidth of the laser beam 178. In this regard, it is noted that only oneof the parent and other CNC's 126 may set the pulse width, with theother CNC 126 adopting the pulse width as set by the selected or PRIMECNC 126. As will be explained in greater detail with regard to thecopending application entitled "PLURAL COMPUTER CONTROL FOR SHARED LASERMACHINING" Ser. No. 414,204, one of the two CNC's 126 is designated asthe PRIME CNC and in effect controls the pulse width and frequency ofthe pulsing of the laser control system 592 of the other CNC. However,each CNC 126 selectively controls the reservoir voltage or outputvoltage from its laser power supply, whereby an individual adjustmentmay be made of the power level of the laser beam 178 as applied to thewelding chamber 108 associated with each CNC 126. Since each weldingchamber 108 is effecting a similar type of weld, a single CNC 126designated as PRIME selects the pulse width and REP RATE, whereas anindividual adjustment of the reservoir voltage or RESVOLT is desired topermit individual adjustment of each of the laser beams directed to eachwelding chamber 108 for the different conditions of the separate opticalpaths, laser lens assemblies 204 and welding chambers 108.

The SELECT GAS FLOW RATE application subroutine is shown in FIG. 25I andis bid by setting a particular M code, i.e. one of M codes M61 to M64,in steps 714 and 730 of the part program shown in FIGS. 24A and 24B, andis executed during the subsequent Bidflag Execute Cycle of the main taskloop or operating system program. Next, in step 932, the three flow rateselect outputs of the optically isolated interface 564 as shown in FIG.22 are cleared, before in step 934, the selected flow rate select outputas applied to one of the mass flow controllers 484, 486, and 488 is setin accordance with the programmed M code. Thereafter, step 936 disablesthe manual or steady state flow rate selection that is set in theabsence of the CNC control.

Referring now to FIG. 25J, the SERVICE S CODE application subroutine isshown, wherein a change of an S code in step 728 of the part programbids the SERVICE S CODE application subroutine to be executed during thepreconditioned departure data cycle of the operating system program. TheS codes determine the laser operating voltage and in particular thatvoltage that is applied to the excitation lamps 186. After being bid instep 950, step 952 determines whether the new S code is within limitsand if not, this application subroutine is aborted and an alarm messageis displayed in step 954 on the CRT 133. If within limits, step 956determines the presence of the M98 flag that is manually set by theoperator to bid in step 958 the CALIBRATION routine as shown in FIGS.25A and 25M. Next, step 960 bids or calls the SET LASER POWER LEVELOFFSET application subroutine as described more fully with respect toFIG. 25L. Finally, the new S code value is transferred to the ProgramControl Buffer (PCB) within the memory of the CPU 560.

The SERVICE T CODE application subroutine is shown in FIG. 25K forcalculating the values of the laser pulse REP RATE or frequency (PRF) asset by the T code in step 728 of the part program of FIG. 24B. Step 970initiates the SERVICE T CODE application subroutine by examining theConverted Data Buffer (CDB) of the memory of the CPU 560. When a changeis observed, the new T CODE is transferred to the Program Control Buffer(PCB) of the CPU memory, and the SERVICE T CODE application subroutineis bid and executed during the preconditioned departure data cycle ofthe operating system program to determine whether the new value of the Tcode is acceptable or not. In step 972, the new value of the T code ischecked with predetermined limits and if outside these limits, theapplication routine is aborted and a suitable alarm message is displayedupon the CRT 133. If within limits, step 974 determines the PRF rangeassociated with the digital analog converter as incorporated within thelaser control system 592. As shown in FIG. 22B, the laser control system592 receives a digital output upon the line marked SELECT PULSEFREQUENCY, which digital output has a corresponding range selected sothat the laser control system 592 may generate a precise analog value ofthe digital output. Next, step 976 calculates the PRF digital/analogoutput value (PRFOUT) in accordance with the indicated equation, whereFSOUT is the fully scale output of the particular digital/analog unit, TCODE is the T code value, and RANGE is the maximum value of each PRFRANGE. It is understood that the calculated value of PRFOUT is appliedvia the SELECT PULSE FREQUENCY OUTPUT of the optically isolatedinterface 562 to the laser control system 592. Next, step 978 bids theCHECK SAFE POWER LEVEL (SAFEPWR) application routine as will beexplained in detail with respect to FIG. 25O. The calculated value ofsafe power is compared with known limits and if outside those limits,the execution of the part program is suspended and an alarm message isdisplayed upon the CRT 133. If safe, the trigger signals as applied bythe optically isolated interface 562 to the laser control system 592 areturned off, and step 986 determines whether this CNC 126 is PRIME, i.e.that the laser pulse REP RATE and the pulse width are selected by thisCNC 126, and if not, an exit is made from this application subroutine.Otherwise, step 988 disables the select pulse frequency output and thereservoir voltage output before step 990 sets the select pulse frequencyoutput with the desired PRF range and percentage of range signals to beapplied via the select power level output to the laser control system592.

The SET LASER POWER LEVEL OFFSET application subroutine, as shown inFIG. 25L, is bid by a change of the S code or of any M code as noted bythe programming of a M code M70 and is executed during the subsequentBidflag Execute Cycle of the operating system program. For example, whenthere has been a change of the S code, the SERVICE S CODE applicationsubroutine is executed and in its step 960, the SET LASER POWER LEVELOFFSET application subroutine is bid. When there has been a change ofthe M code, the M code M70 is set, thereby bidding this applicationroutine. Generally, the SET LASER POWER LEVEL OFFSET applicationsubroutine obtains the reservoir voltage based upon the programmed pulseREP RATE rate as set by the T code and upon the programmed pulse widthstored in a table in the CPU memory. The reservoir voltage is the outputvoltage of the laser power supply 120 that is applied to a pulse formingnetwork, whereby the lamp energizing voltages are generated. In step1002, the offset table within the CPU memory is searched for theprogrammed S, T, and M codes setting the levels of power, frequency, andpulse width to thereby obtain an indication of the reservoir voltage.Step 1004 enables the SELECT POWER LEVEL OUTPUT (see FIG. 22) beforestep 1006 initiates a timing delay. Next, step 1008 actuates the beamswitching mirror 172 to direct the laser beam 178 to the welding chamber108 of this CNC 126a. Next, 1010 checks whether the lamp voltage iswithin limits and thereafter enables in step 1012 the laser triggercircuits, i.e. the TRIGGERS output of the optically isolated interface562 is turned on. Thereafter, step 1014 initiates a time delay to permitthe excitation lamps 186 to reach stabilization before exiting in step1016. Thus, when the PERFORM LASER WELD application subroutine, as willbe described with respect to FIG. 25N, is called, the laser rod 170 isprepared to emit its laser beam 177.

The PERFORM LASER WELDS application subroutine is shown in FIG. 25N andis bid by the M codes M71 and M72 as set in step 728 of the part programand executed during the next Bidflag Execute Cycle. Upon entering instep 1060, the application subroutine Bidflag is set to execute on thesubsequent Bidflag Execute Cycle. After entering, step 1062 determiningwhether one of the laser modes corresponding to one of the codes M51through M54 has been selected. As explained above, the laser controlsystem 592 includes four separate modules, each hard wired andprogrammed to control one of the intersect welds 32, slot and tab welds34, corner seam welds 30, or notch seam welds 40. If no, step 1063displays an "ERROR MESSAGE" before exiting the routine. If yes, step1064 checks to determine whether the GO flag had been previously set instep 882 of the CHAMBER ENVIRONMENT CHECK application subroutine asdiscussed above with respect to FIG. 25F. If not, the step 1066 rebidsthe CHAMBER ENVIRONMENT CHECK application subroutine to again determinewhether the atmosphere within the welding chamber 108 has been purifiedso that its oxygen and water content is below the specified limits. Ifyes, step 1068 actuates the beam switching mirror 172 to direct thelaser beam 178 to the welding chamber 108 of this CNC 126. Thereafter,the dump shutter 190 is disposed to its open position, whereby the laserbeam 177 is directed into the selected welding chamber 108. Thereafter,step 1072 determines whether the M code M71 has been set. As indicatedabove, there are two M codes, i.e. M71 and M72, the code M71 indicatingthat a spot weld, corresponding to the intersect welds 32, is to beperformed, whereas an M72 code indicates that a seam weld correspondingto the corner seam welds 30, the slot and tab welds 34, and the notchseam weld 40 is to be effected. A seam weld differs from a spot weld inthat the fuel rod grid 16 is moved by the X-Y positioning system 288while the laser rod 170 emits a series of pulses of the laser beam 178,whereas a spot weld is effected with the fuel rod grids 16 being keptstationary with respect to the laser beam 178. Thus, if an M71 CODE isdetected, indicative that a spot weld is to be performed, step 1074effects a delay to wait for the X-Y positioning system 288 to come to ahalt before causing the laser rod 170 to fire. On the other hand, if anM72 CODE is programmed, indicating that a seam weld is to be performed,no delay is imparted thus permitting the laser rod 170 to initiatewelding before the movement of the fuel rod grid 16 begins. Next, step1076 checks to determine whether the voltage applied to the excitationlamps 186 is as programmed. Thereafter, step 1078 checks the status ofthe laser and in particular determines whether the temperature and flowrate of the lamp coolant are within specified limits, whether thecurrent and voltage of the lamp power are within specified limits, andwhether the cabinet door 114 is open. Thereafter, step 1080 determineswhether the lamp trigger circuits have been successfully triggered bystep 1012 of the SET LASER POWER LEVEL OFFSET application subroutine asshown in FIG. 25L. If not set, step 1082 displays an alarm message"Trigger Circuit Not Enabled" on the CRT 133. If enabled, step 1084effects laser firing by enabling the shutter control module of the lasercontrol system 592, i.e. applies the start weld signal thereto. Step1086 initiates the timing of a delay period during which the laser rod170 is programmed to complete its lasing, i.e. waits to receive the weldcomplete signal from the laser control system 592. Step 1088 determineswhether a period of eight seconds has expired and if not expired,displays a message "Lasing Completion Time Out" on the CRT 133. Aftertiming out, step 1092 determines whether a spot weld is to be performed,i.e. has the M CODE M71 been set, and if so, the subroutine moves tostep 1096 wherein the CPU 560 generates via the optical interface 562 aRelease Laser Signal on the CNC link 558 indicating that the laser rod170 has been released and that the other CNC 126b may now request thelaser. If a seam weld has been performed, step 1094 closes the dumpshutter 190 and the safety BRH shutter 212, before exiting in step 1096.

The CHECK SAFE POWER LEVEL application subroutine is shown in FIG. 270and is executed to calculate the laser lamp power required for a givenREP RATE and pulse width as programmed by T codes and the M codes M55through M57 and for the laser lamp voltage as determined in the SETLASER POWER LEVEL OFFSET application subroutine as explained above withrespect to FIG. 25L. For the illustrative laser system 102, i.e. theRaytheon model number SS500, the maximum safe power level is in theorder of 16 kilowatts and if the calculated lamp laser power exceedsthat level, the part program is automatically stopped. The CHECK SAFEPOWER LEVEL application subroutine is entered in step 1100 from eitherstep 914 of the SET LASER PULSE WIDTH application subroutine shown inFIG. 25H, step 978 of the SERVICE T CODE application subroutine as shownin FIG. 25K, or step 1026 of the LASER CALIBRATION applicationsubroutine of FIG. 25M. Next, in step 1102, the current T code valuesare obtained from the Program Control Buffer (PCB) of the CPU memory andare used in step 1104 to calculate the pulse forming network factor inaccordance with the indicated equation, where the value of REP RATE isindicative of the pulse frequency as programmed by the T code value, theOFFSET is obtained from the OFFSET table as entered by the SET LASERPOWER LEVEL OFFSET application subroutine of FIG. 25L, and the slope isthat slope as determined from the curve of FIG. 25S. Next, step 1106calculates an intermediate value VSQR as a function of the pulse widthas set by the M codes M55 through M60 and the value of the reservoirvoltage (RESVOLT). If VSQR is less than one, it is known that the lamppower is within the desired maximum limit and an exit is made from thisapplication subroutine. If not, step 1110 calculates a value of thevoltage applied to the pulse forming network (VPFN) in accordance withthe indicated equation, using the intermediate value VSQR, and if BPFNis less than one, an exit is made in that there has been determined thatthe laser lamp power is below the maximum safe level. If not, there isan indication that the lamp power may exceed the maximum safety limitand if so, step 1114 calculates the lamp power in accordance with thevalues of BPFN and PFNFACTR. If the value of the lamp power is below themaximum safe limit, an exit is made, but if not, step 1118 displays analarm message on the CRT 133 and the part program is automaticallystopped. A G register of the CPU 560 is used as a check return indicatorfor the calling routine, whereby upon exiting of the CHECK SAFE POWERLEVEL application subroutine, a return may be made to one of the SETLASER PULSE WIDTH, the SERVICE T CODE, or the LASER POWER LEVEL OFFSETapplication subroutines.

The WAIT FOR OTHER CNC application subroutine is shown in FIG. 25P andpermits the parent CNC 126a to complete a series of welds andthereafter, suspend execution of the part program as shown in FIGS. 24Aand 24B while waiting for a response from the other CNC 126b. Thisapplication subroutine is called by an M code M88 as set in step 735 ofthe part program shown in FIG. 24B and executed during the subsequentBidflag Execute Cycle of the main task loop. This application subroutineprovides a delay in which the other CNC 126b may complete its weldingoperations on its fuel rod grid 16 after the parent CNC 126a hascompleted a particular type of weld and would otherwise change thecommon welding parameters, i.e. of pulse width and REP RATE, as used byboth CNC's 126a and 126b. In this regard, only one of the two CNC's 126is set as the PRIME CNC 126a to thereby control the pulse width and REPRATE, the other CNC 126b adopting the pulse width and REP RATE set bythe PRIME CNC 126; thus, the PRIME CNC 126a must delay resetting itspulse width and REP RATE until the other CNC 126 has completed itswelding operations for a particular weld type. Thus, after step 736determines that a first type of weld has been completed, a check of theM code M88 is made, thus calling the WAIT FOR OTHER CNC applicationsubroutine. After entry in step 1030, step 1032 suspends execution ofthe part program as shown in FIGS. 24A and 24B and therefore maintainsthe same weld type and its REP RATE and pulse width until the other CNC126b has completed its welding operations. To this end, step 1034determines whether this CNC is selected as the PRIME CNC and if yes,step 1040 determines whether the other CNC is currently running. If not,this application subroutine moves directly to step 1048. If the otherCNC is running, step 1044 sets a timing period in which the other CNC126b may set its DONE WELD flag, before in step 1046 setting its WAITWELD signal and initiating a delay. If this processor CNC 126 is notPRIME, step 1036 sets the WAIT WELD for this CNC, while step 1038 waitsfor the PRIME CNC to set its DONE WELD manifestation. In this manner,the delay set in step 1046 is provided for the other CNC 126b tocomplete its laser welding operations before step 1048 clears the WAITWELD flag and then exits.

The GET LASER application subroutine is shown in FIG. 25Q and providesthe means whereby one CNC 126 may communicate via the CNC link 558, asshown in FIG. 22B to bid for the use of the laser system 102. It isunderstood that only one of the CNC's 126a and 126b may have activecontrol over the laser system 102 at any instant of time. Thus, topermit the coordination between the two CNC's 126a and 126b, the firstCNC 126 that is actively controlling the laser system 102 places a LOCKLASER signal upon the CNC link 558. After relinquishing use of the lasersystem 102, the first CNC will place a RELEASE LASER signal on the CNClink 558, whereby the second CNC seeking control of the laser system 102may then impose a REQUEST LASER signal on the CNC link 558. Thereafter,CNC 126 obtains use of the laser system 102 and imposes a LOCK LASERsignal on the CNC link 558, whereby the first CNC 126 is inhibited fromgaining access to the laser system 102. As shown in step 1160, the GETLASER application subroutine is bid by any one of step 1008 of the SETLASER POWER LEVEL OFFSET application subroutine, step 1032 of the LASERCALIBRATION application subroutine, or step 1070 of the PERFORM LASERWELDS application subroutine, and is executed during the subsequentBidflag Execute Cycle of the operating system program. After entry, step1162 examines the CNC link 558 to determine whether the second CNC 126bhas set its REQUEST LASER or its LOCK LASER outputs and if so, step 1164sets the sequence pointer on the first CNC 126a and exits. On the otherhand, if the second CNC 126b has not requested or locked the lasersystem 102, the first CNC 126a places a REQUEST LASER output upon itsoptically isolated interface 562 to the CNC link 588. Thereafter, step1168 initiates a wait for the RELEASE LASER output of the second CNC126b. After the second CNC 126b imposes its RELEASE LASER output uponthe CNC link 558, the first CNC 126a in step 1172 sets its laser busyflag and imposes a LOCK LASER output upon the CNC link 558, before instep 1174 actuating the laser switching mirror 172 to direct the laserbeam to the welding chamber 108 associated with the first CNC 126a.Thereafter in step 1176, the first CNC 126a sets its reservoir voltageoutput, before in step 1178, clearing the routine sequence pointer andreturning to the calling subroutine.

The ROTATE FIXTURE application subroutine is shown in FIG. 25R, and isbid by the M codes M91 through M95 as set in step 728 of the partprogram as shown in FIG. 24B, and is executed during the subsequentBidflag Execute Cycle. After entry, step 1262 actuates the Z-drive motor470, whereby the laser lens assembly 204 is disposed to its homeposition as detected by limit switch 576b. Thereafter in step 1264, theY drive motor 296 is actuated to drive the Y positioning table 292 toits centered position as detected by the limit switch 574d, and the Xaxis is positioned to permit engagement of the B axis rotation drive238. Thereafter in step 1266, the Y drive motor 296 is actuated to drivethe Y positioning table 292 so that the coupling member 362 of thepositioning wheel 358 engages the toothed coupling member 384 associatedwith the B axis rotation drive 238. Thereafter, step 1268 sets the FEEDHOLD to inhibit movement of the X and Y positioning tables 290 and 292,and accesses the outputs of the proximity switches 402a-d to sense thepresent angular position of the rotatable fixture 252 to thus determineif the fixture needs to be rotated, before withdrawing the pin 378locking the positioning wheel 358 and energizing the B axis motor 388.In step 1270, the output of the proximity switch 402d is sensed as astrobe and when sensed, the outputs of the remaining proximity switches402a-402c are sensed to determine fixture position as it is beingrotated to its new position. When rotated to its new position, step 1272deactivates the B axis drive motor 388 as it is being rotated to its newposition, and the trip solenoid 406, whereby the locator pin 378 isreturned to engage and to lock the positioning wheel 358. Thereafter,the FEED HOLD is released and cleared, the Y drive motor 296 is actuatedto move the Y positioning table 292 to disengage the B axis rotationdrive 238, and finally in step 1274, auxiliary processing is enabled,and the routine's Bidflag and sequence pointer of the ROTATE FIXTUREapplication subroutine are cleared.

While not believed necessary for a thorough understanding of the presentinvention, submitted herewith and made a permanent part of the filewrapper of this application is a computer program listing showing how aconventional computer as identified herein may be programmed to simulatethe various functions of the above described application subroutines; tothe extent that this material is deemed by the Examiner to be necessaryunder the provisions of §112 of the Patent Laws, it is incorporatedherein by reference.

In considering this invention, it should be remembered that the presentdisclosure is illustrative only and the scope of the invention should bedetermined by the appended claims.

We claim:
 1. Computer controlled apparatus for performing a series oflaser machining operations on a work piece in an environment comprisedof a gas non-reactive with respect to the material of which the workpiece is made, said laser machining apparatus comprising:(a) a machiningchamber for receiving the work piece and for receiving the non-reactivegas to establish a machining environment about the work piece; (b) anexcitable laser for generating laser emission; (c) optical means forfocusing and directing said laser emission as a laser beam along a pathto the work piece; (d) means for directing the non-reactive gas intosaid machining chamber at a selected rate; (e) means for selectivelymoving said machining chamber and the work piece in a sequence ofmovements with respect to the laser beam, whereby each of the machiningoperations is performed on a selected site of the work piece; and (f)programmable computer means for controlling the laser machining of thework piece and including first means for controlling said gas directingmeans in accordance with a programmable parameter indicative of saidselected rate, and second means programmable with a set of instructionsindicative of the machining sites of the work piece for controlling saidselectively moving means to sequencially move said machining chamber andthe work piece disposed therein in said sequence of movements.
 2. Thecomputer controlled laser machining apparatus as claimed in claim 1,wherein said first means is programmed with a first parameter forcontrolling said gas directing means to provide the non-reactive gas tosaid machining chamber at a first rate sufficiently high to evacuate anygas reactive with the material of the work piece from said machiningchamber.
 3. The computer controlled laser machining apparatus as claimedin claim 2, wherein said first means is programmed with a secondparameter for controlling said directing means to direct thenon-reactive gas into said machining chamber at a second rate less thansaid first rate, said second rate being sufficient to maintain thepurity of the non-reactive gas within said machining chamber to ensurethe integrity of the laser machining operations effected upon the workpiece.
 4. The computer controlled laser machining apparatus as claimedin claim 3, wherein said first means is programmed with a thirdparameter for controlling said directing means at a third rate less thansaid second rate to permit the removal of one work piece from and thereplacement of another work piece into said machining chamber.
 5. Thecomputer controlled laser machining apparatus as claimed in claim 1,wherein said programmable computer means comprises third means forcontrolling said laser to generate said laser emission.
 6. The computercontrolled laser machining apparatus as claimed in claim 5, wherein saidthird means controls said laser to provide laser emission, while saidsecond means controls said selectively moving means to move saidmachining chamber and the work piece to effect a seam type of weld. 7.The computer controlled laser machining apparatus as claimed in claim 5,wherein there is included means associated with said machining chamberfor measuring the reactive gas within said machining chamber, and saidprogrammable computer means comprises fourth means for evaluating theoutput of said measuring means for inhibiting said third means fromcontrolling said laser to provide laser emission if the content of thereactive gas within said machining chamber exceeds a predeterminedlimit.
 8. The computer controlled laser machining apparatus as claimedin claim 7, wherein said first means is programmed with a firstparameter for controlling said gas directing means to provide thenon-reactive gas at a first rate sufficiently high to evacuate anyreactive gas from said machining chamber, and said fourth meansevaluates the output of said measuring means and if the content of thereactive gas exceeds said predetermined limit, said evaluating meanscauses said first means to operate said directing means to direct thenon-reactive gas into the laser chamber at said first rate.
 9. Thecomputer controlled laser machining apparatus as claimed in claim 8,wherein said fourth means evaluates the output of said measuring meansto determine that the content of reactive gas within said machiningchamber is less than said predetermined limit to cause said third meansto initiate the operation of said laser to generate and direct the laserbeam onto the work piece.
 10. The computer controlled laser machiningapparatus as claimed in claim 1, wherein there is further included beamdirecting means disposed to intercept the laser beam and operative in afirst mode for directing the laser beam along the first mentioned laserpath, and in a second mode for directing the laser beam along a secondlaser path distinct from said first laser path, said first mentionedmachining chamber disposed to intercept said first mentioned laser path,a second machining chamber for receiving a second work piece anddisposed to intercept said second laser path, second optical meansassociated with said second path for focusing the laser beam onto thesecond work piece, and second means for selectively moving said secondmachining chamber and its second work piece with respect to said secondlaser path.
 11. The computer controlled laser machining apparatus asclaimed in claim 10, wherein said programmable computer means comprisesthird means operative for disposing said beam directing means to operatein one of its first and second modes to direct the laser beam onto oneof the first and second work pieces, while said second means actuatessaid selectively moving means associated with the other of said firstand second work pieces to move the other work piece in preparation forthe next laser machining operation.
 12. The computer controlled lasermachining apparatus as claimed in claim 11, wherein there is furtherincluded second programmable computer means for controlling the lasermachining of the second work piece, and said first-mentioned and saidsecond programmable computer means operatively coupled to said beamdirecting means for disposing said beam directing means to operate insaid first and second modes respectively, said first-mentioned and saidsecond programmable computer means coupled to each other to permit onlyone of said first-mentioned and said second programmable computer meansto gain control of and to operate said beam directing means in itscorresponding mode, while excluding said other programmable computermeans from gaining control of said beam directing means.
 13. Thecomputer controlled laser machining apparatus as claimed in claim 12,wherein each of said first-mentioned and second programmable computermeans comprises said third means for enabling and inhibiting its controlof said beam directing means, and link means coupled between said thirdmeans of each of said first and second computer control means.
 14. Thecomputer controlled laser machining apparatus as claimed in claim 13,wherein each of said third means is responsive to its gaining control ofsaid beam directing means for applying a laser lock manifestation tosaid link means, whereby said third means of said other of saidfirst-mentioned and second programmable computer means is inhibited fromgaining control of said beam directing means.
 15. The computercontrolled laser machining apparatus as claimed in claim 14, whereinsaid third means of one of said first-mentioned and second programmablecomputer means generates a laser request manifestation on said linkmeans, whereby said third means of said other programmable computermeans is inhibited from gaining control of said beam directing means.16. The computer controlled laser machining apparatus as claimed inclaim 15, wherein said third means of one of said first-mentioned andsecond programmable computer means applies to said link means a laserrelease manifestation indicating that said one programmable computermeans has completed its laser machining and that the other of saidprogrammable computer means may now gain control of said beam directingmeans.
 17. The computer controlled laser machining apparatus as claimedin claim 1, wherein there is further included rigid bed means resting ona support surface for defining a first substantially planar referencesurface, means affixed to said first reference surface for rigidlymounting said laser with respect to said first reference surface, aplatform, work piece housing means affixed to said first referencesurface for defining and disposing a second, substantially planarsealing surface with respect to said first reference surface, saidmachining chamber having an upper opening defining a third substantiallyplanar peripheral surface, said machining chamber being disposed on saidplatform, adjustable means for positioning said platform and saidmachining chamber mounted thereon so that said third peripheral surfaceis disposed substantially parallel with respect to said second sealingsurface and spaced therefrom a distance sufficiently small to ensure theintegrity of the machining environment within said machining chamber,and fastening means for affixing said platform to said work piecehousing means once said third peripheral surface has been so positionedwith respect to said second sealing surface.
 18. The computer controlledlaser machining apparatus as claimed in claim 17, wherein there isincluded a laser subbase defining a fourth substantially planar surfacefor receiving and mounting said laser, rigid frame means affixed to saidfirst reference surface for mounting said laser subbase, and meansaffixedly mounted on said frame means for adjusting the position of saidlaser subbase so that said laser beam is precisely directed onto thework piece.
 19. The computer controlled laser machining apparatus asclaimed in claim 1, wherein there is further included a slide table formoving said selectively moving means and said machining chamber betweena first position, wherein said machining chamber is disposed to receivethe laser beam onto the work piece therein, and a second position,wherein said machining chamber is removed from the laser beam to permitthe work piece to be disposed within and removed from said machiningchamber.
 20. The computer controlled laser machining apparatus asclaimed in claim 19, wherein said machining chamber comprises an upperedge defining an opening for receiving its work piece, and there isfurther included sealing means disposed at a substantially uniformspacing from said upper edge to provide a peripheral gap between saidsealing means and said machining chamber to facilitate the movement ofsaid machining chamber without restraint as imparted respectively bysaid selectively moving means and said slide table.
 21. The computercontrolled laser machining apparatus as claimed in claim 20, whereinsaid sealing means comprises a first substantially planar surfacedisposed to form a substantially uniform spacing with respect to saidupper edge.
 22. The computer controlled laser machining apparatus asclaimed in claim 21, wherein said selectively moving means moves saidmachining chamber and its upper edge along paths that define a second,substantially planar surface substantially parallel to said first planarsurface.
 23. The computer controlled laser machining apparatus asclaimed in claim 22, wherein there is further included means responsiveto the laser beam to provide a manifestation indicative of the power ofthe laser beam, said measuring means mounted upon said slide table andsaid slide table is movable to a third position, wherein the laser beamis aligned with said measuring means.
 24. The computer controlled lasermachining apparatus as claimed in claim 23, wherein there is includedmeans responsive to a control signal for exciting said laser to emit itslaser beam of a power corresponding to said control signal, and saidprogrammable computer means comprises third calibration means coupled toeach of said exciting means and said measuring means, said thirdcalibration means being responsive to the measured power of the laserbeam for providing the control signal.
 25. The computer controlled lasermachining apparatus as claimed in claim 24 wherein the laser machiningoperations include machining with a laser beam generated in accordancewith at least first and second sets of distinct lasing parameterscorresponding to first and second machining modes, respectively, whereinsaid programmable computer means comprises fourth means for programmingsaid computer control means with said first and second sets of lasingparameters.
 26. The computer controlled laser machining apparatus asclaimed in claim 25, wherein said exciting means excites said laser toemit the laser beam in a series of controlled pulses.
 27. The computercontrolled laser machining apparatus as claimed in claim 26, whereineach of said first and second sets of lasing parameters includes aparameter indicative of the desired power level of the laser beam. 28.The computer controlled laser machining apparatus as claimed in claim27, wherein each of said first and second sets of parameters includesparameters indicative of the pulse width and repetition rate of thecontrolled laser pulses.
 29. The computer controlled laser machiningapparatus as claimed in claim 28, wherein said programmable computermeans comprises fifth means for comparing the measured power of thedirected laser beam and the value of the programmed parameter indicativeof the power level to provide a signal indicative of the differencetherebetween.
 30. The computer controlled laser machining apparatus asclaimed in claim 29, wherein said computer control means comprises sixthmeans responsive to said difference signal for adjusting said controlsignal, whereby the actual power level of the emitter laser beam isadjusted accordingly.
 31. The computer controlled laser machiningapparatus as claimed in claim 1, wherein there is included means fordirecting radiation onto said laser of controlled frequency, wherebysaid laser emits a series of laser pulses of corresponding frequencyalong said path, shutter means disposed to intercept said path andoperable between an open position to permit the laser pulses to bedirected onto the work piece, and a closed position to prevent the laserpulses from being directed onto the work piece, said programmablecomputer means comprising third energy control means for actuating saidshutter means to its open position for a period of time to permit awhole number of the laser pulses to be directed to the work piece,whereby the laser energy imparted to the work piece may be preciselycontrolled.
 32. The computer controlled laser machining apparatus asclaimed in claim 31, wherein said programmable computer means comprisesfourth means for entering parameters indicative of the whole number, thepulse width of and the repetition rate of the laser pulses applied tothe work piece to provide pulses of laser emission of sufficient peakpower to progressively weld the work piece to the desired penetration.33. The computer controlled laser machining apparatus as claimed inclaim 31, wherein said third energy control means comprises countermeans for counting the number of laser pulses applied to the work piece.34. The computer controlled laser machining apparatus as claimed inclaim 33, wherein said third energy control means comprises switch meansfor applying a selected count to said counter means, whereby saidcounter means counts to said selected count before disposing saidshutter means from its open to its closed position.
 35. The computercontrolled laser machining apparatus as claimed in claim 33, whereinsaid third energy control means comprises timing means for generatingand applying a series of timing signals of a selected frequency to saidcounter means to be counted thereby.